We need pictures. And price will vary based on who you are talking to. A good welder (or someone really willing to learn) who sucks at mechanical would like that car since his skills can fix what is wrong and vice verca if it was a great shell with a blown motor. Also what are the good points? did he add any aftermarket/go fast parts like headers, ignition stuff, etc?
We need pictures of the rust to get a better idea anyway, but I would talk him down.