Just curious if I’m missing something… is there no trial period or, say, a development key that doesn’t return live data?
It would certainly be helpful to have so I don’t kill my meager budget just trying to develop a simple app! 
The service is charged exclusively by usage, there’s no set up or monthly fees to consume your budget. With the FlightXML2 pricing a query at less than a penny we don’t see sufficient value in creating and maintaining a service to provide stale data for free and have concerns about the data accidentally being used in production environments.
500 queries for development will cost you less than $4, and I think we don’t charge credit cards until the balance reaches $5.