for me, the Jrok was the most expensive part ($80).
next, was the jamma harness; pre-wired $30 (I think)
then a power supply; you can use a computer ps, I went and bought an arcade ps for about $30.
I used a Line level converter for hooking up sound through RCA jacks to my stereo, I got it at a local electronic store $15.
and of coarse the joysticks. I modified 2 Saturn pads, they cost me $5 for the pair.
odds and ends: wire, connectors, etc. etc. <- maybe $10 (I had a bunch of stuff laying around the house)
and finally (I think), the enclosure you want to put it all in. I used a horizontal computer case, painted it up all pretty. It looked alright, but my next supergun will look much better!