GunFodder said:
http://www.fakecrap.com/pulling-trick-p/1120.htm
These are just about the easiest and cheapest traps I can imagine.Perfect for trip wires, chests, doors, or anything. VERY loud.
http://www.milsimlabs.com/parts.htm
At the top of the page are essentially pressure traps. Load a cap gun cap into them, place anything over 1/2 pound over it, and when you lift it up about 3/4 of an inch, it goes off.
There's no reason why every trap-making PC shouldn't have dozens of either of these ready to go. I'd look at your local costume/magic shop (that's where I got mine), they're a bit more expensive online.
I like using the pull poppers for triggers on traps I don't want to go off by themselves. It takes a bit of force to pull the string so you can usually feel the tension before it goes off. The pressure traps are always nice as long as you can actually disable them. I've seen quite a few interesting traps that were 'hair triggers' that even those who set them couldn't disarm them.
The problem isn't having the trap reps, the issue is that they are rather ineffective overall. If you want to deal with a trapped area, have someone strip down and walk through. Once one is set off, feed them a cure light wounds potion/elixir. Rinse, repeat.
Using them during combat is all but useless as you have to call a massive hold to find out who was in range, and usually those that were won't be there by the time the hold is called. That is if anyone hears the trap go off in the first place.
What would be nice is if there was more 'physical' traps. Such as one that blew baby powder, sprayed water, fired foam pellets, etc. Anyone hit takes the damage. Unfortunately between high safety requirements and fear of ruining costuming, such traps would be hard to get passed.
However, I don't think that it should cost any less than it does right now. A proper usage of it can take out an entire wave of baddies if proper set up and execution before combat ensues is done right. Explosive traps, even if they don't do enough damage to kill everyone, will destroy all non-magical items (weapons, armor, gas globes, etc) and there is no way (that I'm aware of) that you can avoid the damage as long as you are in that 5 foot radius.
My mini tavern was wired with a half dozen explosive traps, with only the far corner behind the bar safe from the overlapping radii (and with a trigger to one of the traps). Set one off and they all go off and you suddenly have a bunch of people without any equipment and taking anywhere from 30 to 150 points of damage (depending on where they were sitting).