Dungeon Economy

How much does it cost to build a dungeon or wizard tower? For those featured in adventures, this is simple: who cares? The writer does whatever he damn well pleases, and ignores the rules that apply to players. It's magic. It does not have to make sense from an economic perspective. This is a game of high adventure, not accounting. Just take a look at Rahasia, chock full of teleporters, golems and magical statues, and the most powerful magic user in the place is level 4 (level 1, as long as the witches are not forming a coven). And even for PCs, the DM may decide to handwaive some costs, to drive the story forward, and provide the adventurers with a basis that then gives them a stake in something that can be attacked.

That said, nobody in their right mind would spend more to protect his property than what it is worth. Therefore, the costs for guards and traps need to be adequate for the value of the treasures and building. It would be absurd to protect a treasure worth a few hundred gp with a vault that costs thousands. 

So what is a way to equip and secure a place that makes economic sense? To answer this, we must know what spells, labor and mundane items cost and what they do, which differs by game system. I would like to have this for D&D 5e, which is not well suited for this, because it does not provide power-calibrated prices for magic items, nor any prices for many of these other items. So let's see what it would cost. We can get inspiration from OD&D on tricks and traps we could use.

Inofficial dungeon building costs for 5e 

• For magic items, I am using my own and Sane Magic Item Prices, with crafting costs of half those. 
• For dungeon furnishings, OD&D/1e, brilliant as ever, has prices for much of what one could wish for in mundane costs, from dungeon doors to pits and all the way to whole towers. From a comparison of prices for all armor, weapons and mundane items in 1e and 5e, the value of a gp in 1e is about 1.25 times that of 5e. That is close enough for me to just use the price list from 1e. 
• For Spellcasting services cost spell level squared times 10 gp (plus material cost - DMG uses double material cost). 
• For Mundane items listed in the 5e PHB, such as a simple lock, cost is as listed in the PHB. 

Installing most items assumes skilled craftsmen, at a cost 2 gp/day. An unskilled laborer costs 2 sp/day, and is included in the item cost for simplicity. Hiring skilled NPCs like guards scales the cost linearly by CR, starting at 2 gp/day for CR 1/8, to 16 gp/day for CR 1 and 24 gp/day for CR 3.

Difficulty Classes and Masterwork items

Some effects, like locks or traps have a DC, and the default is 15. As a general rule, to increase the DC by 1 point will cost 80% of the base price, up to DC 20. To increase it above DC 20, double the price for each step. This assumes it is very difficult and costly, to create such masterwork versions. The DC cannot be increased over 25 without magical support or a legendary craftsman. For example, a simple DC15 lock costs 10 gp. A DC 20 lock would cost 50 gp. A DC 25 lock would cost 1,600 gp. (Alternatively, you could add a flat fee of 100 gp to any base price item as a simpler rule, but that would fail to reflect the underlying base costs).

Permanent Magical Effects

There is no Permanency spell in 5e. Some spells have built-in ways to make them permanent, but this leaves holes for many of the effects you are looking for. My solution to permanency for outfitting a building is:

• Permanent magical effects are crafted like magic items, based on their cost. During the crafting time, the spell needs to be cast every day, including any material costs.  A single caster can craft 25 gp worth per day, each assistant may accelerate this by also crafting 25 gp worth, without need for an additional casting or material components. 
• Like with  Glyph of Warding, permanent spells cannot be moved from their place of creation. They end if moved more than 100' away. This keeps them local to the dungeon, tower or room.
• The caster must be of the level to craft the level of spell, as for spell scrolls (DMG p. 200).
• Crafting cost is 10 times the price of a scroll (i.e. 20 times the cost to craft a scroll).
• Permanently crafted spells are magic items, and as such cannot be simply dispelled with Dispel Magic. You may deduct 10% from the base cost if it can be surpressed this way for 10 minutes.
• Instantaneous effect spells cannot be made permanent, but a  Glyph of Warding triggering them can, e.g. for a recharging Fireball trap. Recharge is after 24 hours. Faster recharge is more expensive: double each step per hour, minute, round, anytime the trigger is fullfilled. 

The price table for spell scrolls (and thus, spell permancency) is 

Level     Cost

Cantrip   25 gp 

1st          50 gp

2nd       150 gp 

3rd        300 gp 

4th        600 gp 

5th     1,500 gp 

6th     3,000 gp 

7th     6,000 gp 

8th   12,000 gp 

9th   24,000 gp

Teleport effects: these have a built-in condition like Glyph of Warding. They do not require the caster, have no mishap chance, cannot teleport into solids, need to place you on stable ground (not lava, water etc.), and by default have a Charisma saving throw. They trigger for base cost anytime the condition is fullfilled, and get more costly with each slower recharge step. Without saving throw, costs are doubled. Teleport within 90 feet counts as a level three spell, within 500 feet as a level four spell, within the same plane to a teleport circle as a level five spell, within the same plane anywhere or to another plane as a level seven spell. If you can teleport into air or over water, lava etc, allowing for a reaction, costs are doubled, directly into lava, tripled. (These are relatively cheap to reflect how commonly they are seen in adventures, if abusable, could add a cost multiplier).

Walls: if you have a wall such as Wall of Force or a Wall of Fire to block access to somewhere, you likely want to be able to supress it so you can pass. As a wizard, you might just Dimension Door past, but a more convenient way would be to have some trigger that supresses the effect for a short time. Adding such a trigger adds 20% to the cost. 

Examples: a permanent Unseen Servant would cost 500 gp (compare to the salary of about 73 gp per year for an unskilled laborer). A short range teleport that triggers anytime someone enters the room without save would cost 6,000 gp, one sending them to another plan without save 120,000 gp. A Wall of Force sealing a treasure chamber that you can inactivate would cost 18,000 gp. A Gate to hell would cost 240,000 gp - maybe a little too cheap, that's less than half of what a mundane castle costs.

Spells that are typically useful to make permanent are

Cantrips: Light, Dancing Lights, (Continual Flame ist cheaper for light), Minor Illusion 

Spell Level 1: Alarm, Fog Cloud, Grease, Unseen Servant, Silent Image (upcast Major Image is cheaper)

Spell Level 2: Darkness, Gust of Wind, Levitate, (Silence)

Spell Level 3: Animate Dead, Tiny Servant, Magic Circle, Glyph of Warding

Spell Level 4: Conjure Minor Elementals, Mordenkainen’s Faithful Hound, Wall of Fire

Spell Level 5: Animate Objects, Conjure Elemental, Summon Greater Demon, Passwall, Wall of Force

Spell Level 6: Arcane Gate, Create Undead, Globe of Invulnerability, Wall of Ice

Spell Level 7: Mordenkainen’s Magnificient Mansion, Plane Shift, Reverse Gravity, Teleport

Spell Level 8: Antimagic Field, Antipathy/Sympathy, Demiplane

Spell Level 9: Gate, Prismatic Wall

Alarms

This includes everything that alerts the owner, his allies or his guardians if there are unwanted intruders. Alarms can be built as mechanical traps in the form of trip wires, bucktes full of bells over doors and such. 

The classic option is the Alarm spell, that even a low-level mage can maintain if they are refreshing it. Magic Mouth is the best option for a permanent alarm, or you could craft Alarm permanently, if you want it to be silent. 

Higher level alternatives include a Contingency to trigger a Message or Sending to himself, if the GM allows for it, or a servant triggering a Glyph of Warding that contains Sending, and responding with whatever message he wants to send to you.

Blocks

This includes everything that restricts access, from a simple, locked dungeon doors, portcullis or barred windows, to a massive vault doors and permanent walls of force. Prices from the 1e DMG:

Wooden Door   (4'w, 7'h)     10 gp     3" thick

Wooden Door, reinforced     25 gp     4" thick with iron bands

Iron Door  (4'w, 7'h)           100 gp     1" thick

Portcullis (10'w, 15'h)         500 gp    

Lock, simple                         10 gp    DC15, add to cost of door

Extra cost calculation for thicker iron vault doors scales with thickness: if it is twice as thick, it will cost double.

The default spell for magical enhancement here is Arcane Lock, cheap at 50 gp and permanent, at higher levels there is Antipathy. Or permanent magical walls, especially Wall of Force.

Traps

This includes everything to stop, scare, harm, weaken or kill intruders, in case your access controls are not effective. There are no prices for traps (other than a hunting trap for d4 damage and 25 gp) in the rules.  Default DC to detect and disarm is 15.

Poison Needle     poison   25 gp non resetting (small in locks of chests etc)

Dart trap                  1d4    40 gp non resetting (often poisoned)

Arrow trap             1d10    75 gp non resetting

Spear trap              3d10  250 gp non resetting

Pit, 5' cube                           4 gp walled and floored with finished stone 

Trapdoor 2,5'w x 2,5'          2 gp, double if secret, add 1 gp if spring-closing; add lock if locking

Triggered traps can be reset by a skilled worker with 10 minutes of effort. Self reloading versions fire once per round, cost thrice the price, and need to be restocked when the magazine of 20 projectiles is empty.

Instead of going over all the individual traps (scythe, boulder, acid, etc.) individually, price them by damage, with an increment of 100 gp for each d10. This gives a discount to the common pit, bolt and spear traps, and explains why they are so common.  Poisons Gas fills a 10 foot cube for the price under Poisons. Sleep gas with Drow Poison stats is available.

Poison prices. I lowered the price of basic poison to 25 gp, as it deals so little expected damage, even at that price, it is not a great buy.  Poison of common poison creatures (normal Spider, Snake, or Scorpion) is common, that of giant variants or humanoids under 500 gp is uncommon, above 500 gp it is rare. Poison of devils, demons, or above 1,200 gp is Very Rare, and above DC20 it is legendary.

For magical traps, Glyph of Warding sets the standard, at a base cost of 200 gp for self installed spells. See above about permancy to make it reusable. Other common traps include permanent teleporters. 

Misdirection

This includes illusions, secret and hidden doors and hidden compartments. 

Hidden Door                     free*

Secret Door (2'w, 4'h)        50 gp    DC 15

Illusory Wall (5'w, 5'h)    250 gp    (permanent Minor Illusion)

Illusory Wall (20'w, 20'h) 360 gp   (permanent Major Image)

* Hidden doors do not cost much extra and are a great low-cost option. If you have curtains or gobelins hiding the door, or put your cupboard in front of it, to only cost would be that of the furniture.

Illusions may also be of monsters to fake-out danger and scare off intruders. They are best combined with real effects for enhanced believability.

Commonly spells are permanent Minor Illusion, as well as Major Image and Programmed Illusion. Illusory Script and Nystul's Magic Aura can be used to disguise magical writing and items.  Guards and Wards is a multi-purpose protection spell and can be made permanent by recasting it for a year and 3,650 gp.  

Abjuration

Protection against magical intrusion. Coating some rooms or chests in lead may defend their contents from magical detection. A one-pound lead ingot is about 2 cubic inches, cost 1sp. Three can cover a square foot with a thin sheet.   

Lead Sheeting (5'x5')        7.5 gp

Common spells are Mordenkainens Private Sanctum and Hallow. Mordenkainens Magnificent Mansion and Demiplane create spaces that are inaccessible. 

Guards

This is the most flexible and complex option. It includes hired guards, trained or charmed animals and monsters, retainers and allies. Magical companions work too. Undead, summoned monsters, magical constructs, and creatures put in stasis are good if you do not want to have long term maintenance costs or have no way to supply your guards. 

The game limits the ability to create such long-term allies, because if you could do so, and take them on adventures with you, this would be unbalancing. So there are only few, and often costly ways to do so.

Living guards

Allies and friends may be the best and cheapest way: if you have retired adventuring companions you have someone you know you can trust. By providing them with free lodging, you gain a powerful defender in case of an attack. Occasional presents from your adventures will be appreciated. In our case, we have a goblin ranger henchman, a mid-level cleric, and an adult gold dragon living with us. We gift them with treasure, gems and minor magic items when we stop by, to keep them happy. 

If you have an Homunculus or a familiar with a long range connection to you, these can work very well by secretly hanging around, observing what is going on, and warning you. 

The simplest option is to hire normal NPC guards. As skilled hirelings they cost 2 gp/day, or 730 gp/year for challeng rating 1/8. I'll reduce this with the rationale that long term secure emplyoment requires a lower rate than uncertain day wages. Untrained commoners have no combat training and will flee from any danger, but can act to stable-boys, servants, etc. NPCs are smart, which is worth something, as many magical servants are not.

CR   Hireling   Cost/year

0       commoner    50 gp

1/8    guard          500 gp

1/4    acolyte     1,000 gp 

1/2    thug         2,000 gp (reskin as guard sergeant)

1       spy           4,000 gp
2       captain    8,000 gp (bandit captain, reskin as guard officer)
3       veteran  12,000 gp

These values provide a benchmark for magical creations: more expensive in absolute, cheaper over time. 

Trained monsters, such as guard dog mastiffs, or wolves or lions, are also great.  They have keen senses, and only cost their food as maintenance, but eat expensive meat. Snakes both giant and poisonous, sharks etc are all possibilities, but are to stupid to train and hence not listed. More exotic monsters such as mimics, or chimeras may be found on adventures and charmed or kept at peace by feeding (GM adjucates the cost). As they lack higer intelligence, you need to combine them with a skilled keepers that can tend to them, if you cannot do so yourself.  If you can charm them and transport them, this still is very cheap for a lot of dangerous creature.

CR    Beast   Cost/year

1/8    mastiff  100 gp

1/4    wolf      150 gp (also: panther, leopard)

1       lion       600 gp 

Spells like Speak with Animals and Animal Friendship for druids can help to enlist many small, innocous spies and informants that make it difficult to enter unobserved. 

Undead

Animate Dead and Create Undead are your bread and butter spells here. Zombies make guardians, but are too dumb for much else. Skeletons on the other hand are surprisingly intelligent, and could act as simple cooks, manservants and so on, as could the unsavoury ghouls. Ghasts and Wights are as intelligent as humans, and can manage your affairs. The halfwit mummies are not worth it. 

The main downside is that you have to be present to recast the spell every day, or these creatures will become unbound and do how they please.  Of course, Necromancy is despicable, and you will get distrustful looks if you practice it openly. 

Binding them permenently means crafting. The cost for a skeleton is about three years' of wages for a guard, so worth it if you have a long term perspective.  For the high end undead, wights are the best, they are as smart as humans and as tought as mummies at half the cost.

CR    Monster    One-time cost

1/4    skeleton    1,500 gp

1/4    zombie      1,500 gp

1       ghoul       10,000 gp

2       ghast        60,000 gp

3       wight       60,000 gp

3       mummy 120,000 gp

Summoned Creatures

Conjuration spells for devils, demons or elementals (including mephits, xorn, invisible stalkers; djinni and efreeti are of too high a CR to be summoned) are high level spells starting only with spell level four and hence are the domain of powerful wizards, or of wealthy patrons who can afford their services. 

Summoning alone is insufficient for long term service. One either has to strike a bargain or pact with the summoned entity, or one needs Magic Circle and Planar Binding, upcast for a longer term duration, the maximum being a day and a year at level nine, which requires the services of an archmage. These things can go wrong, many a wizard has been killed this way. Any pact should be at least twice as costly then the corresponding coerced option.

If the owner can do all the spellcasting himself, the cost is 1100 gp. If you can do 7th level Bindings, the annual cost will be 13,200 gp a year. That is still very cheap for a CR 6 guardian. (For creatures with Dispel Magic or teleportation to get around the circle, there are extra costs). 

If you have the services of an archmage there are additional costs of 900 gp for binding in a circle, and the conjuration itself, for example for an invisible stalker 360 gp, total cost 2,360 gp/year. Note that invisible stalkers are usually only to hunt down and kill.

CR    Monster                                                    

2        gargyole

5        air/water/fire/earth elemental  

5        salamander

5        xorn

6        galeb dur

6        invisible stalker

Unless you want the creature as your majordomo or are naturally friends with such a creature (see above), a more economic alternative for summond creatures may be to put them into a Sequester or summon them with Glyph of Warding. 

Constructs

The least morally questionable solution for long-term servant. You can find examples in the Monster Manual: Animated Object (Sword, Armor, Rug),  Helmed Horror, and for high challenge ratings the Golems (Flesh, Clay, Stone, Iron). Scarecrow feel too much like a horror flick, maybe if you are evil. Unfortunately through all the editions, there is no simple animated statue for lower challenge rating of tier one play. Dungeon of the Mad Mage has a Living Statue), Icewind Dale a Snow Golem, Ghosts of Saltmarsh a Living Iron Statue, Mordenkainen's Tome of Foes clockwork creatures, Bronze Scout, Iron Cobra, Stone Defender, Oaken Bolter, and Sacred Statue. There are no rules on how much it costs to create any of them. 

To limit abuse they are bound to only function inside and near the building. If you want them unbound, double the cost. If you buy them ready made, double the cost again. If you allow a Dispel Magic to entirely undo them, not just suppress their animation for 10 minutes, reduce the cost by 25%.

CR     Cost

¼        2,000 gp 
½        4,000 gp 

1         8,000 gp 
2         16,000 gp 
3         24,000 gp

Crafting based on permanent Animate Objects would be a lot cheaper, so that spell is excluded from being made permanent. Crafting Golems requires a very rare Manual of Golems that is consumed, see there for their costs (a cheap 65,000 gp to 100,000 gp). 

Sequester

Sequester will cost you 5,000 gp, and allow you to stash away any creature for one-time release. As such, it is well suited to release a powerful guardian, say, a dragon, and that at a price that can be way lower than the CR that can be achieved for ongoing guardians. However, it is not useful for ongoing stewartship or patrolling of your premises.

Price List Dungeon Trappings