$300 for a washer?

I have a plumber doing some work on my tenant's bathroom. They got it all clogged up. Nice how they remove the strainer in the tub so the hair doesn't clog everything up. There is also a leak on the bathroom sink just above the trap. Plumber tells me it will cost $300 to replace the trap. I ask if the trap needs to be replaced and if he can tell me what is causing the leak. He says that is what they do, replace the whole thing whether it needs to be replaced or not, or whether just a washer is cracked. Hmm...washer is probably half a buck tops. Or I pay him 300 bucks to replace the whole thing. How stupid does this guy think I am? Ridiculous. He needs to take the trap off anyways to clean out the pipe. If it is a washer that is causing the problem, and he puts it back on there, I will be pissed. I have used these people many times and if they want to screw me on this, I will be finding a new plumber. So much for keeping a customer happy.