The cost of animal health care is as crazy as people health care. It can get out of hand in a hurry. Either way, if you don't have the money, you are just stuck. It's about impossible to know what the 'right' choice is sometimes.
If it is a tooth, then treatment might make him good as new. He's an old man though; if it is a tooth, it might be way bigger than that and be several teeth; that is, a lot of work would be needed. You can't know whether it is one tooth, five teeth, or a tumor without the diagnosis. That step should give a you much clearer information. Whatever it is, pain-wise it is likely to get a lot worse in a hurry. My best guess is that you'll have to make some decision immediately.