The plumber finishes the job and gives him the bill and the customer says, "I'm a surgeon and I don't make this much money."
The plumber says, "I know. I didn't make as much either, when I was a surgeon."
This is not to say the bill was reasonable. The OP should see if a new valve was installed, and then could call another plumber and ask what his price would be.
Then he could call the plumber who did the work, and say he thinks there was a mistake on the bill, blah, blah, blah. If he says, I think your rates are criminal and he wants a refund of part, the guy will just get angry. I've never tried my plan either, but you never know.