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?