Why don't you ask around, and see what/if people are charging to check. If they all charge about the same, then it's reasonable. If they don't charge, then no it's not reasonable.
But if I remember correctly they need to be changed at 90K miles, so you shouldn't need a check, you need it replaced. When you go in, don't say you want your belt checked, say you want it replaced.
You shouldn't wait for something like that until it fails, you should replace it before it fails. Yeah, you can go 120K miles or more on it, but they recommend you do it at 90K miles so you prevent any damage caused by it failing.