How much should this repair cost?

Is the radiator bad? If we had more information on his diagnosis, it would help.

Also your city (or the prevailing hourly wage in your area) and his estimate would be helpful.

Cannot see all three going out at the same time. But. The thermostat housing (chicken pooh plastic) costs about a hundred bucks, and is changeable by yourself, as it the thermostat and new sending unit. which are not that expensive