why would he take you for just a belt? too many other possibilities, like, maybe it needs the fan bracket, tensioner or otherwise to make the belt run correctly? those plastic pulley do age and maybe they were missed by the tech.

as far as parts, it's part of the gross profit the shop needs to maintain overhead, just like the eggs in a diner. all things considered, your opinion and his reaction, maybe try once more. if that doesn't work, then find a new shop. good luck, chuck.