Hmmm sounds abit pricey but the use of a car is obviously included in that price. If it was just a stock universal can like a Jap style can which are imported in from the far east and are cheap as chips its abit dear, but if its a custom built back box from scratch with a choice of tailpipe options then its not that bad really.
I paid £275 to have a manifold fitted and a custom cat back system fabricated with a custom back box
If your happy with the price and your happy with their quality of work then does it matter how much you pay?
Id say go for it