[RPG] What are the rules for crafting traps that don’t cost anything


In D&D 3.5, crafting time is linked to the cost of the item created. But how is it handled when the item doesn't have a cost? Example: a group of PCs want to build a camouflaged pit trap to kill some monster. It is location trigger (no cost), with repair as a reset (-200gp), there is no bypass, the search DC shouldn't be more than 20 (no cost), if not less (-100gp x (20-DC)) as it is made with improvised materials (think branches, dirt, moss leaves, anything found in the wild). The same goes for disable device.

This leaves the PCs with a trap that costs maximum -200gp to create.

I don't think it would make sense to tell making it was so easy they went back in time, or to tell them they finished instantly. So how do I handle it?

Best Answer

Mechanical Trap Minimum Cost

It doesn't look like you're pricing that trap correctly. Here's from the mechanical trap rules:

The base cost of a mechanical trap is 1,000 gp. Apply all the modifiers from Table: Cost Modifiers for Mechanical Traps for the various features you’ve added to the trap to get the modified base cost.

The final cost is equal to (modified base cost × Challenge Rating) + extra costs. The minimum cost for a mechanical trap is (CR × 100) gp.

And from the trap creation rules:

The base CR for a mechanical trap is 0. If your final CR is 0 or lower, add features until you get a CR of 1 or higher.

By the rules, you can't create a trap below CR 1, and the minimum cost is 100g.

Trap Math

Lets look at your costs compared to the rules. The base cost is 1000g.

It is location trigger (no cost)

Correct, this is no change to the base cost.

with repair as a reset (-200gp)


there is no bypass

No modification to cost.

the search DC shouldn't be more than 20 (no cost), if not less (-100gp x (20-DC))

Correct, so a search DC of 15 would be a modifier of -500. Total of -700. You have no extra costs, as there's no alchemy involved.

Your cost is calculated as such:

The final cost is equal to (modified base cost × Challenge Rating) + extra costs. The minimum cost for a mechanical trap is (CR × 100) gp.

So you have (1000 - 700) x 1 + 0 = 300g. You could get it down to 100g if you lowered the save DC on the pit to 18 instead of the base 20.

Note that you currently have a CR 0 trap, and the rules require minimum CR of 1. You can get to 1 without increasing the cost by adding average damage (adding 7 average damage increases CR by 1). This would not increase the cost.

What if it really didn't have a cost?

If something really doesn't have a cost, you can make something up. For "digging a hole in the ground", the cost is time. The PCs can do it themselves, or they can hire someone. An untrained labourer costs 1sp per day.

The economy in 3.5 doesn't make a ton of sense, so you can assume the extra cost in a trap vs a hole in the ground is what's required to actually fool anybody into ever falling into it. It's best not to try to expect too much logical consistency on this stuff, becuase unless you use houserules, it doesn't exist.

How Long Does Digging a Hole Take?

Races of the Dragon p.98 has rules for using Profession (Miner) to mine, which you could also use for digging a big hole for your trap. The rules cover how long this would take based on how good a check you can make, with modifiers for different type of ground. You could combine those with the hireling cost to get an idea of what it'd cost to hire a bunch of people in town to dig a 10x10x10' hole.

(Summary version: a medium size miner can empty a 5' cube per day with a DC 15 check. DC 10 would do half of that, and every 5 you beat it over 15 lets you do another 5' cube. Large size works twice as fast, small half as fast. A second medium miner can assist and use aid another to boost the role, while smaller creatures can have more assistance.)

Now that you have your hole, you'd just have to figure out how to conceal it. There's no particular rule for that outside of the trap rules that I know of, but you could make something up.

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