If the price seems high, you could replace the parts your self if you know how.
And call the plumber out to reinspect.
You may even order the parts and have them on site, and just pay the plumber to install the parts and retest.
But, there is a bit of skill involved with RP repair.
For some good information on my specific niche of choice try my website for pictures and information on backflow.
BackflowInformation.com