That might be a very reasonable price. Some of these are all electric with built in sensors and three actual valves. Very expensive stuff. Before spending the money you might try cleaning the valves as they do get carbonned up and stick some. Doesn't take much to cause surging and the engine service light to come on. One good thing though, the whole EGR system is in the valve so replacing it will fix the problem for sure.
I don't believe the dealer is pulling a fast one on you, most are pretty reputable. If you're fixing this yourself, try around at a few other parts stores first. There's a few different aftermarket brands out there that may have that available. You can also try looking for a parts store that advertises AC-DELCO parts. The dealerships rarely charge anyone less than list price, but I have seen cases where the price was increased due to attitude.