Recently, the pentagon decided to cut the 65th Aggressor Squadron, the United States Air Force F-15 unit that mimics enemy fighters like the Russian Su-27. This squadron was used for training, playing the adversary role in dogfights.
The reason was the cost. According to Time Magazine, the cost per flight-hour for some military jets is gigantic. The F-15 costs roughly $41,921 per flight hour. (This includes fuel and maintenance costs.)
By comparison, the F-16 costs $22,514.
Some European fighter jets like the SAAB JAS 39 Gripen have costs closer to that of the F-16.
The A-10 Warthog costs $17,716 per flight hour. (It is also the subject of a budget battle at the Pentagon and may be cut to help fund the new F-35 Joint Strike fighter.)
Smaller drones like the Predator and Reaper drones cost $4000 or $5000 per flight-hour.
These costs do not include the costs of the bombs and ammunition.