when i moved out of home and into a small flat, my neighbours didn't like the noise from my drums, so i sold my acoustic set and built a digital set.
i bought an alesis dm5 drum unit, and a practice pad kit. went to tandy's (radio shack in australia) and bought myself some piezo's, drilled a hole in the bottom of the practice pads and hot glued the piezo's inside. my mate soldered some jacks onto the piezos and bob's your uncle.
it didn't allow for dual triggering, and i already had a double kick pedal, and although it wasn't the prettiest of things, it was fantastic. the most expensive part was the dm5 which was about $400 from memory (but that was brand new). i had many years of playing on that kit virtually noise free, until i decided to upgrade my motorcyle, and sold every musical instrument/amp that i owned (except for my acoustic which i have had for over 25 years - no real value save for sentimentality).
here are some links for sites that tell you all about building pads.
Link 1
Link 2
Link 3
it is so basic and simple, that i think it's criminal the amount of money digital drum manufacturers charge for their pads.
cheers
darren