Copper pipe and fittings cost big bucks. The likely reason this sort of plumbing is leaking after years of operation is that the system and plumbing wasn't properly winterized. Water freezes and plumbing breaks. Keep that up, and you will have big repair bills every spring.
If the PVB part of the plumbing doesn't leak, then the bad plumbing and valves can be replaced. You can also just replace the entire PVB-with-valves with a new one. This all costs money, and the quote you got was not unreasonable.