An M1 Abrams tank cost

[catmandu57]

4,300,000 according to http://www.fas.org/man/dod-101/sys/landm1.htm An Apache helicopter will set you back either 16 million http://www.antiwar.comapache/.html or over 56 million if you go by http://www.nationmaster.com/encyclopedia/Apachehelicopter.
I haven't been able to come up with the unit cost of humvees and heavy trucks that are regularly targeted in Iraq.
The CBO is estimating the war costing 6-9 billion PER MONTH. It's not going to take long to run through that 87 billion, especially if the iraqis become proficient at taking out tanks and helicopters.

Another problem, that's been discussed before but is worth mentioning again, the military's hardware is wearing out. They didn't bring enough spare parts to begin with, and it's an awfully long supply line.

So it may be six months or sooner and the military will either be ready to mutiny, or turn into the Roman army and get really mean, but either way it's not going to take long to run through the money.