I had to go buy $35 in lumber (plus $5 in wood screws) for this beaut, but the real expenditure was on the wonderful controller I got from http://treyonics.com/. These guys are great! Forget about that cheesmo X-Arcade controller; take a look at what Treyonics' Devastator II offers on their website...you NEED 2 joysticks, a trackball, a spinner, 6 buttons for each joystick (in-joystick button optional), plus buttons on the side for pinball games.
Here are pictures of my "finished product"...it ain't pretty, but it's pretty fun: http://devlinks.net/gallery/MAME_cabinet
Total build time: 3 days.
Total cost (assuming no cost for crap that was sitting around): $490
"Right now I feel that I've got my feet on the ground as far as my head is concerned." -- Baseball pitcher Bo Belinsky