Figuring the costs of Iraq...help!

[tedzbear]

The U.S. Iraq war budget was around $80 billion and the occupation is costing roughly $4 billion a month.

I am confused to how they arrive at these costs. Since most of the military costs are salaries and upkeep that would occur even if they had never left the United States, what the hell is costing the extra $80 billion?
The only added expense I can think of is the transportation costs to ship everything over there, and the replacement value of the weapons dropped on the Iraqis's heads. What am I missing???