How often should you clean the trottle body on a car? I got 26000 kms on my car and the dealer wants to charge me 150.00 dollars to service it.Am I getting ripped off here? He says it's part of the warranty.

That's a little high. What they'll be doing is a top end induction cleaning. It's good for the car, but no warranty requires it.
Just ask to have the throttle body and plate cleaned which should be done every 30K miles. They do get pretty gunked up.

Heck, You could do this yourself. Take you about 30 minutes. The throttle body plate gets pretty gummed up. When your car is at idle it is barelt open. When it is gummed up you are interferring with the air flow. Get a old toothbrush and a can of high quality cleaner. You will be amazed at how much better it will idle, much less run.