Plumber is overcharging you? I should say up front that I am a plumber, so you are right to read this with one eyebrow up. But I wrote it anyway, because I spent years at a big Valley plumbing company and I watched how the sausage gets made.
Here is the uncomfortable part. At a lot of companies, the person at your door is not paid to fix your problem. He is paid on what he sells. There is a weekly dollar number he is supposed to hit, and a script to help him hit it. He is usually not a bad guy. The incentive just does the work. If you pay someone to find expensive problems, they find expensive problems.
So here is what to watch for. None of these alone proves anything. Two or more together, and I would get a second opinion.