What would you give....

If you get a chance, hook the tank to pressure from the other compressor to check for leaks. The second test is unhook the belt and turn the pulley on the head, you should be able to tell if I makes air or it's locked up, if it turns freely it should be rebuildable. The last thing is the motor, even after a few years you should be able to smell the electrical burnt smell if it is fried. To answer your question no more than a few hundred depending on the problems that it has.