I bought a low-cost e-kit a couple of years ago and was surprised at the simplicity of the individual drums. The trigger itself is just a piezo that you can buy in most electronic-stores for about 1€, and if you are relatively comfortable with a soldering-iron, it’s quite easy to connect to a jack-connector to it, and you’ll have a drum-trigger, not very practical thou, the hard part is building something that can hold it steady against a drum/mesh-head.

I tried building my own trigger-mounts for mounting inside the drums, but working with metal or wood is absolutely not one of my strong sides.

A more simple approach is to make something like Roland’s BT-1 Bar Trigger Pad.
It’s just a block of rubber with a piezo attached, and a mount with a female jack, I used double-sided adhesive tape to glue the rubber, piezo and foam together and to the board.

This works on all the modules I have tried from Roland, Alesis and Millenium, and I’m sure that someone with more experience in metal/wood-work can make these themself for a fraction of the cost, and most likely a bit more steady than mine.

Now you just need a drum-module to turn the piezo-signal into MIDI and/or sound, any module will do if you just need the MIDI signal to control a sampler, or some another hardware like a CMX light or even a DAW.

I remember seeing somewhere that it’s possible to buy piezos with a presoldered female jack, if you want to avoid soldering completely.

You could even build a drum-module yourself using a microcontroller like arduino. It’s a bit more complicated, but if anyone is interested, I would love to make a howto for a Arduino-based MIDI-only drum-module.