What are the rules for crafting traps that don't cost anything?

Question

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?

Answer 1

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)

Correct.

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.

Answer 2

Crafting a Pit Trap

There are a wide variety of Pit Traps, but based upon your presentation you are talking about a relatively simple hole that only servers the prupose of capturing the target. There are questions you need to ask the players concerning their process:

1.) How are they digging the hole? Shovels, mage hand, move earth?

2.) What are they doing with the dirt and rock they removed? A heap next to the hole, taking the time to cart it away, concealing the heavily worn trail from dirt removal?

3.) Where and how are they gathering the materials to cover the pit? From near or far and are they using Wood Shape or Fabricate or just collecting branches? If the branches are from too close tehy will be leaving all kinds of evidence that something has disrupted the environment around them, if they are gathering it from sufficient distance to not be incriminating that will take time to collect.

4.) How are they concealing the pit so that it blends in with the surrounding ground? A loose leaf covering over branches in a dirt trail would be obvious. They could be using mundane methods or magic, illusion to help conceal the trap.

5.) Did they do anything to conceal their passing, cover the scent of their having been there? Pass without a trace, oitments, balms to cover their scent.

6.) What are they doing to make sure their target is not observing their opperation? It seems they are laying a trap where they expect their target to travel, do they know when it will pass or if it might come by early.

Everything they do takes incriments of time and they need to take into account the abilities of their prey, such things as sense magic, sense of smell, visual acuity, intelligence, the things that effect how they do what they do. Planning the job takes time and you can use the actual time the players take to discuss the project as their planning time. Using magic to do things usually takes less time than the manual tasks, but you could use that for a measure if minimal times, but if the creature can sense magic, using it would blow any concealment. There is definately room for skill checks and craft checks. The pit trap wont do a lot of good if the creature is large and caves the side of the pit into a slope before it actually falls in or if the branches call in a manner that allows the creature to climb them to get out. Crafting and skill checks will be more difficult considering the materials are improvised and failed attempts extend the time taken to accomplish an end result. A failed attempt on laying the cover may result in a cave-in or other consequences that may be detrimental to the objective.

Assuming you were looking for time adjustments, not gp cost, there is also search or spot checks for finding materials that are useful, appraisals of materials to see if they are sufficient to the task, endless things you can use to string the time and difficulty out. If the sticks are too old and rotted the trap will collapse before it has served its purpose, if the cover is too strong the target may walk across or successfully spring off before it collapses. The end goal would be to give it enough challenge to keep the player interest and you know your players well enough to keep it fun.