Military aircraft dont get much use in peacetime, at least compared to commercial versions of the same aircraft. So in wartime you can pump up the usage quite a bit. But it costs you more in the form of fuel, spare parts and maintenance personnel. That last item is another problem. Putting lots of hours on the helicopters and aircraft is one thing, getting your hands on enough pilots and ground crew to keep up this high tempo is another problem. You dont hear about all the extra aircraft maintenance personnel going to Iraq, but there they are and they are being worked hard. The air force also has a policy of keeping its people in Iraq for only four months at a time, so theres the additional expense of getting them in and out. This policy means the hard working aircraft maintainers get out to recharge their batteries, and not overwork to the point where they can make a fatal error.
The cost of operations in Iraq is up to a little over a billion dollars a week. This is higher than expected, mainly because the government has not been able to organize new Iraqi police and security units as quickly as they are needed, and the opposition in Sunni Arab areas has been more aggressive in their use of terror against Iraqis, and ambushes on American troops. The result of this has been much increased use of helicopters, and other aircraft. This makes life easier and safer for the good guys, but it costs a lot more money. Running an AH-64 Apache helicopter gunship costs over $4,000 an hour. Even a UH-60 transport helicopter costs over $2,000 an hour. While there are not a lot of bombing missions over Iraq, most will be run by smaller aircraft, like F-16s, which cost $5,000 an hour, versus a B-1 bomber, that costs over $10,000 an hour. But various combat, transport and electronic warfare aircraft based on the C-130 get used a lot more as well. These aircraft cost $2,000 to over $5,000 an hour to run (depending on how many people and gadgets they carry while working.)