Trust The Plan Evangelist
This doesn't make any sense. It's not like the vast majority of a plumber's customers are asking for cpvc specifically, a plumber could get away with using pex themselves very easily if they wanted to and could still charge whatever it is they want to charge.
A plumber's money comes from labor, not materials upcharges.
PVC doesn't cost more than pex really. It takes longer to install though, and is easier to fuck up. You can make it look nicer because it goes in straight lines, but in some ways that's bad for the customer (more elbows, longer runs because you can't go places diagonally, so you have to run out more cold water before the hot gets there, etc. Also, nobody looks at plumbing except for plumbers so the straight lines and such don't matter to most customers even if it's a point of pride to the plumber. Where you really hurt the plumber is by not hiring him at all, which is easy to do with pex.
Ironically everyone wants to charge more for pex. The $6000 quote (for a 2/2 small cabin) wants 10% more to install pex which I found fucking weird. They said it was more work since they needed to secure it in more places so it didn't knock the walls when valve went off.