CSIS, 8 Aug 07: The Uncertain Cost of the Global War on Terrorism
...If Congress approves the FY2008 war requests, the Congressional Research Service (CRS) estimates that total funding for Iraq and the Global War on Terror would reach about $758 billion, including about $567 billion for Iraq, $157 billion for Afghanistan, $29 billion for enhanced security, and $5 billion unallocated. According to the Congressional Budget Office (CBO), the Defense Department is currently obligating an average of almost $11 billion a month for expenses related to its operations in Iraq and Afghanistan and for other activities related to the war on terrorism. Most of that sum (more than $9 billion per month) is related to Iraq.

The CRS found that the estimates of expenditures being issued by the U.S. government understated the real cost of operations because they included only current obligations, not the cost in terms of worn equipment, future replacements, paying the long-term costs of increased pensions and payments for killed and wounded, and paying for a long list of additional costs such as support by the national intelligence agencies.

There is no way to do more than guess at the ultimate cost of the Iraq War, the Afghan War, and the war on terrorism. CBO projected the cost of war for the 2008-2017 period under two scenarios. The first case assumes a more rapid drawdown of the troops deployed in Iraq, Afghanistan and other places in support of GWOT operations; the cost in such a situation would range from $481 to $603 billion, depending on the duration of the surge in Iraq. Under a second scenario, assuming a more gradual drawdown, the extra cost for the next decade would be between $924 and $1,010 billion, again depending on how long the current level of troops is maintained in Iraq....
CRS, 16 Jul 07: The Cost of Iraq, Afghanistan, and Other Global War on Terror Operations Since 9/11