The way dealerships work is they give you a quote for JUST the belt price + labor for installing the belt alone. It's usually a LOW amount to get you to agree to the work... If you press them for more info then they will often give you the REAL quote that includes the total cost of replacing the common parts they often find along with the belt replacement itself.
Once you agree to the low quote, the mechanic gets into the engine and they already have it taken apart (so you going to pay for the job anyway) they start inspecting every component and if anything from the tensioner to the water pump, idler bearings, etc. show any sign of wear (which of course they will after 80,000) then the service tech will call you and say "Hey, we were replacing the belt but we noticed your tensinor is bad and the water pump was bad and we need to replace those parts as well." At that point, what are you going to do?? Say NO?? If you do then they will begin to talk you into the service saying that they cannot guarantee the work unless you allow them to replace everything they find that needs replacing... bla bla bla
You will then be charged for each additional part itself + the labor charge to install that part and it adds up in a hurry.
In the end, you'll end up paying between $500 - $800 for the job by the time you are done.
Or you could take your car to the corner garage and have them only replace the belt for $300 and then 8,000 miles later something else will break and you'll need a new engine. This is why some people seem to have constant breakdowns, they take their cars to mechanics who don't fix everything at once, they only fix the parts you tell them to fix and then within a month something else breaks that SHOULD have been fixed with the previous repair.