Have you gone to any other places to get quotes to compare? That's the first step I would take if I wanted to know if the place you got the quote from was unreasonably high.

If it is one of the top fab places they will probably be charging more than a guy who does it on the side.

There is a fair bit of labour in doing what you want. From what I've read on other sites to get a vehicle tubbed is usually around $10k from a shop drive in, drive out.