Most shops charge like $100-200 for a single piece because it's a lot of work. When you spray the whole car you don't have to worry about colors matching exactly. Just sand, tape, and spray. When it's just one piece they have to mix the paint, apply it to something, let it dry, verify it matches, sand the part, (if it's still on the car) tape, spray, blend, etc. It's harder to get an exact match, and they charge you for it.