Motor mounts can absolutely be that expensive depending on how they have to be replaced. In some vehicles, you can merely hoist the engine up a bit to remove the mounts. In other vehicles, you practically have to take the engine out to get to the mounts. Now, having said that, rarely do all 4 mounts break at once. Generally it's only one mount, perhaps 2. There is no need to replace any mount that is not broken. That's a waste of money.
Have you had your Check Engine light on in the van? A defective O2 sensor will trip the Engine light. Generally the way a mechanic knows it's the O2 sensor is because they have run the computer codes. Did your mechanic run your van's codes? If he did - WHY did he when you came in for a throttle body service? (There ARE other symptoms, but they can be confused for other causes.)
So far as the induction service goes, it's basically just a fuel system cleaning. The 2 things you get out of it (usually) is a fuel injection cleaning and a (ta-dah!) throttle body cleaning. Now, this can be good if you're getting ratty gas and subsequent bad gas mileage. Most of the time though, it's not necessary. I would recommend a tune-up and a bottle of Fuel System Cleaner from your local auto parts store before a fuel induction service.
Personally, I never do business at a dealership. I've found that their service tends to be priced much higher than a general mechanic and they push services that just aren't necessary. I would take the vehicle to a local mechanic with a good reputation and ask for a thorough inspection. Let them know that you were told you needed motor mounts and ask for a separate quote. You can quite easily price the mounts yourself by calling NAPA or another local auto parts store. Labor can only come from the mechanic himself.
Good luck!
