[TylerE]

Well, don't discount fuel.

I had the oppurtunity to spend an hour in a T-6 trainer a number of years ago. This was the primary trainer used by Air Force in WW2. Big radial engine. At full throttle (which we were mostly at, as we were doing acrobatics), that eats through fuel at about 40 gallons/hr. That's $200/hr just in gas, for a plane that carriers 2 people and isn't really all that powerful.