Here's a good condition DINAN E30 M3 for 15 grand:
http://cgi.ebay.com/ebaymotors/1988-...#ht_500wt_1182
That's a bit much. For one of those you could probably find one (non-Dinan) for around or less than $10k.
But since you're looking for something cheap, you can probably get late 80's 325i's (in decent condition!) for less than $5000, probably only a couple thousand if you're willing to take one that's not in the best shape. You just gotta keep checking craigslist and ebay regularly until one pops up. You never know when someone will feel like listing it!