If you are talking about the water pump belt then $90.00 is a bit high but it is something most people can not do, you need to consider that!
Look at where it is and try to figure out how to replace it. When you buy your own parts and want a shop to install them for you, it's like taking your own hamberger meat to Mcdonalds and asking them to cook it for you! That has a negative effect on shop owners and labor prices tend to be higher!!