The problem with any paid API is that the API consumer needs to make money somehow so that he can offer the price of the service.
How will the API consumer make money? I don't think Ads will generate enough revenue -- especially not on mobile.
Kayak will get some revenue from the bookings resulting from the searches, and would need to circle back some of this revenue back to the API consumer. This is technically complicated because it requires a lot of tracking infrastructure.
In the end, a service that would make sense is a lot more complicated than simply having an endpoint and saying it's X dollars per Y calls.
