Does the pressure valve on the wall need to replaced
Having replaced the water heater in my home recently, and also having a surprise needed replacement of the pressure reducing valve too, I can sympathies with your situation.

If you are just replacing the Pressure-Reducing valve for your home, you can find Pressure-Reducing valves from $50 to $150 at your local big box home improvement store. For our project we spent around $95 to match the size of the threads to the previous arrangment. 

Also when working on hot water heaters and valves, I learned that this is also the time to have a Expansion tank installed, or replaced. If this is also something in the agenda, you are looking at around $40-90 at the same retail outlet. 

Also factor in cost for copper / pex, etc fittings, and any tools that you need and there should be your cost to do it yourself.

Depending on the plumbing in the home, it could take anywhere from minutes to hours (like mine) to replace. I'd recommend having someone confident in the task to complete the job. It's possible to do it on your own, just make sure to do your thorough research.

If possible, request a itemized list of expenses for the project to help you understand the high cost. If unable to be provided with that, I'd recommend shopping around for another plumber as this does seem high.