fixing my ford escape?

I have a 2010 ford escape. The car is driving along and then wont accelerate any more. The check engine light come on and sometimes the wrench light comes on. I had a guy from our city garage run a scan on it. I cant remeber the exact code but it said throttle actuator stuck open, or something closes to that. Today I called and the deal said my warranty wont cover that. Two questions, why wont the warranty cover it and How much does the throttle body cost. It looks like it is just 4 bolts and no problem to change