As an X-Air Force pilot I know that every squadron has a monthly flying time budget that it meets to the last tenth of an hour. If they can do a stadium flyby within the budget it doesn't cost anything. At Reese AFB Texas they did flybys for Texas tech football games and it didn't change the budget a penny.
But the T-Birds and Angels road trip is another thing altogether. Those road shows cost huge money. In a time of belt-tightening to bounce the budget, air shows are a good place to cut. Also a round trip to Hawaii in a 747 for a round of golf.