Some groomers are just not equipped for a dog that size. Maybe they don't have enough help to lift him into a tub, or they have no cage large enough to house him for drying. They might just not be able to physically DO a large, heavy breed (it gets a LOT harder as you get older).
As for the price, $65 and UP depending on condition of the coat/behavior of the dog. Some coats are a lot thicker than others, and some owners don't have a clue that their dog is not brushed out properly at home. And a dog that behaves at home for a brushing might put up quite a fuss at the groomers (or vice verse)
As for petsmart.....it will depend on the groomer more than the salon. While pet store grooming shops tend to have lesser experienced groomers, that is not always the case.