More than possible to make a cab for > $300. my two player layout came to just over $200. Of course, there is no trackball or spinner, but that's something I can add in the future.
Cab - $0 (except gas and tolls)
Monitor - $89
Computer - $0 (scrounged from machines I found)
Motherboard - $15 (the original one blew)
Buttons/Joys - $0 (scrounged from CPs I found)
KeyWiz - $40
Wiring - $0 (CAT-5E found in Staples dumpster)
I know I'm forgetting something in there, but I can't remember what.
Now for the stuff I screwed up and still had to lay out $$$:
Primer: $5 (wrong kind (cheap stuff))
Marquee - $11 (printed on vinyl, not see-thru, will have to redo)
Keyboard - $7 (I can't solder worth a damn)
Paint - $unknown (I keep screwing up the paint job, having to repaint over and over)
It's a decent machine. 500MHz AMD K6/2 with 96M RAM. Decent in the respect that I can play the games that I want. No Killer Instinct, but this machine isn't made for that. It does play the Neo Geo games decently. I've grown to like the Metal Slug series.