This is kind of a general overview, the basic concepts are fairly simple. I looked at a lot of info out there before building my own, and I just kind of planned it as I built, it just takes a little creativity. Sorry to not include any links, just google it, I couldn't find the specific pages I used, but there is a community of people out there who do this stuff.
So an electronic drum set can run you back $600-3000+, sometimes without a module, my main reason for doing this was to save money bigtime. For comparison the cost for me was around $150-200 for all parts, then the module, so a total of at most $370, which as you drummers know is even cheaper than entry level acoustic sets! The most expensive item was the electric drum module or heart of it all which I will get to later. Heres a quick summary of my bill of major materials and costs:
-Drum pads -> 2 used toms 10" and 12" about 20$ each
-Cymbals -> plastic practice cymbals, $30 for a set
-Bass pedal/stands/mounting hardware -> came from existing acoustic set FREE
(note: I had these laying around, I would suggest being a bit creative about your solution if you dont own a drum set. I had first planned on using pvc or steel pipe, it would be much cheaper as a drum stand can run up to about 100$ for a simple studry one, Perl,Tama etc. The bass pedal might be the only thing you need to purchase, check for used hardware etc, or again, this is Instructables, your crafty people :-P).
-1/4" mono Wires (one per pad) & Electronic parts, $30-40
-Wood -> FREE (scavenged)
-Drum Module, $170 (ebay, "Alesis D4")
Basically I'm trying to convey how little money you would actually have to spend to have a working set minus the drum module. This set, after tuning the sensitivity of the pads (function of the drum module), gets about 4-5 levels of "volume" depending on the power of the hit. It's a great set for practice, and I wouldn't hesitate to play it live if I got the cash for a nice amp together...