Change tempering valve on hot water system

You need to know the brand used to get any sort of an accurate answer but most valves are between $80 to $120 or more. Plumbers also load the price because if something goes wrong with the part, they have to cover it. Did the plumber leave the box behind? You cannot complain about the mark up but you could have asked for an itemised account.

Many plumbers have a call out fee and sometimes a minimum time charge for a small job. A call out fee (if charged) and a minimum time charge (usually 1 hour but sometimes more) plus the mark up could easily total $380.

You should always get quotes...did you do this?

There are some plumbers over on the Whirlpool Home forums, you should also ask on there.