I can tell you exactly how much this costs because I just had my tire done at good year. It was $28 total (counting all taxes etc).
It probably depends where you are located also as small towns probably do it cheaper if you live in a small town. It was pretty quick for them to do this so its not a long wait assuming they get you in right away. If you have alot of tread left on the tire then I think I would just patch it for sure, if the tire is bald, forget it, just get a new tire and dont waste money patching the old tire.