Lots of variables here not the least of which is location and actual condition. Very hard to put a hard number on it as it is a bit subjective, however a roller (w/o drive train) that is very rusty runs $3-6K, so a complete car that is salvageable (aren't they all) I would say $6 - $18K+. I am pretty cheap and I already have or have had pretty much everything I would want, so my estimations are probably off. I just looked at eBay and there are a few on there running between $7-$18K (none are "buy it now"), so its hard to say what you could actually get one for.
I know this isn't overly helpful but its about all I can offer at this point. Guess my advice would be "as cheap as possible".