I don't think you have to change the belts on these cars earlier than the scheduled mileage unless 1) you didn't perform oil changes as frequently as required or 2) you drove all the miles in stop and go city traffic that puts extra wear and tear on the engine. When you do replace the belts it is a good idea to replace the water pump because as the dealer said it is already at hand and will save you future labor costs. $750 is about right for a dealer, but it will cost quite a bit less at an independent mechanic. I would go to Car Talk’s Mechanics Files site at
Enter your zip code and you will get a list of mechanics in your area that other consumers have used and highly recommend. Look for ones that specialize in the make of your car. Call 2-3 of them and ask for a ballpark figure on the repair. I have personally used this site with great success to find good honest mechanics and can highly recommend it. Good luck!