While dozens of vehicles are destroyed in Iraq each month, that is not the major vehicle expense. In this year and last, $550 million is being spent to replace lost vehicles and equipment. But $5 billion is being spent on vehicle and equipment maintenance in Iraq and Afghanistan. Another $6 billion is being spent on repairs (at all levels, from the unit to depot.) These costs are more than five times the peacetime level. This shows two things. First, the troops don't use their vehicles that much in peacetime. Part of the reason is cost, the other is the need to keep as many, as possible, ready for action. Second, combat vehicles, and military equipment, is designed to take a lot of punishment. But that means the stuff is built to high standards, and replacement parts are expensive.
When the vehicles are used heavily, in harsh (as in hot and dusty) conditions, under combat rules (hot rodding, cross country, explosions and gunfire), lots of expensive spare parts are needed. For all those armored hummers, the extra weight and combat conditions has burned out engines and busted suspensions. Radios and other electronic gear take a beating as well. So it should come as no surprise that you spend twenty times as much on maintenance and repair, as you spend on replacing destroyed stuff.