I missed the opportunity to buy on eon ebay for $25. I still kick myself for missing that one.
i won an auction for a 4-player door for ~$26 with mechs that looked in pretty good condition and had an atari sticker on it. if you keep checking ebay you will occasionally see them pop up. others have gone the 2 2-player route, which is common on simpsons and tmnt cabs as well..
found this link in the FAQ, (which i would definitely recommend if you hadnt read it twice already) for a cheap 4-player coin door:
http://www.videoconnect.com/pages/miscsupply.htm"COIN CONTROLS COIN DOOR (NEW) - w/4 entry slots, locks & key. Fits standard 10" x 12" openings $49"