How many hours of labor are you being charged and at what rate? To do the job right would include the tensioners and a crank seal and cover gasket. They are possibly also having to remove the slam panel, grill shell, radiator & intercooler for access. Also, if they are following the SM proceedure, they have probably had to invest in a pump alignment pin and a crank alignment pin to set everything properly....so maybe 4ish hours at $70-80/hr plus parts & shipping & tax...I can see it coming to $600 pretty easily.
I put a belt in the truck I just converted. I did it without a radiator & stuff installed and it wasn't overly difficult but it would have been a real PITA to do with the radiator in place.