What do I owe the mechanic?

I took my car in for new tires and found out I needed to replace my subframe (correct term?). Anyway the price was in line with what I was willing to pay because I want my car to last. While doing the work a bolt snapped and the price to repair that moves things into the realm where it makes more sense to get a similar used car. If I choose to refuse any further work what do I owe for work already done? Can I just ask the mechanic to junk the car? If it makes a difference, this is a major chain. I'm not talking about putting a small business in a difficult situation.