I've heard an ineresting rule of thumb on the E30 M3. You are looking at a range of $15-20k. If you find one that doesn't need work in great condition, you'll pay near the top of that range. If you find one for, say, $8k, but it needs work, you'll end up putting $7-12k to get it into great shape.
Good luck with your search. I've always thought it would be awesome to DD a classic M.
BMW CCA member - Peachtree Chapter