What it does is try to work out the frequency of the note you are playing on the guitar, then produce a sound at that frequency but with a tone that sounds like a theremin. The tracking is a bit temperamental, so it makes a few 'woo woo' noises as well, but that adds to it I think. It can only do single notes at a time, and works best with a neck pickup and highest strings. Not so good with bass notes. Also not good with fast playing, slow songs are best to let the tracking work.

The PCB is 8 euros I think, plus a bit of shipping. Parts weren't too expensive, it has a few ICs, but they are common ones you can get from Jaycar or RS or similar. Check out the link for the PCB, it has a build document that will list the parts. I can't remember the details but I'm guessing less than $20 parts?

The echo is something I put together special, so it has a few buttons to warp the sound, etc, but you can probably get an echo PCB from most places that sell diy pedals. Or just leave off the echo, just do the theremin fuzz by itself.

The case is an old Apple Airport base station (wifi router). Not sure where you can find one of them - the one I found was down the back of my parents' filing cabinet. It's probably about 15 years old.

Really the 'expensive' bit of building something like this is your time.

If you wanted a pre built one, the guy who makes the PCB also sells them finished (but in a more traditional enclosure). I think he also has kits with all the parts included - check out his site?