The price all depends on what you want done and what fabric you want to use. If you want to use cheap vinyl and skimp any place you can, the price will be lower. That's why you should find out EXACTLY what your upholsterer is going to do for the price he is quoting. When I re-do door panels, I'm talking about replacing the old panels and mounting clips, not using what was there. I never do that, even if they seem to be in good shape. The same thing goes for the kick panels and package shelf. When I re-do seats, I'm talking about repairing or replacing any spring problems, re-padding the seats and replacing the old seat covers. (I absolutely do not understand why any upholsterer would make new seat covers and put them over the old springs and padding.) That's the only way I do things, so I will refuse a job rather than have a customer tell anyone else that I did his crappy interior.
It's just like anything else, you get what you pay for.
No one lives forever, the trick is creating something that will.