President Bush's budgetary request in the supplemental spending bill passed in late May included a funding estimate to cover the surge of additional troops only through the end of FY2007. CRS points out that military appropriations allots funding for types of expenses--such as personnel costs--rather than for specific operations, advising that, "Unless Congress enacts specific restrictions, the president can use currently available DOD funds to conduct military operations including the deployment of additional troops."
The report also outlines different restrictions Congress could adopt to affect military operations and troop levels in Iraq. According to CRS, Congress could cut off funding for particular types of military activities but permit funding for other activities (e.g., prohibiting funds for combat activities but permitting funds to withdraw troops); cut off funds as of a certain date in a specific country; cut off funds “at the earliest practical date,” which essentially gives the president leeway to set the date; cut off funds if certain conditions are met (such as a new authorization) or certain events take place (such as the release of U.S. prisoners of war).
The Democrat-led Congress has been pressing for Bush to set a schedule for the drawdown of US forces since they assumed power, but recent weeks has seen even senior Republican allies of the president calling for a new strategy in Iraq. The latest CRS estimates of the running costs of the war may put even more pressure on a White House struggling to maintain the upper hand in directing Iraq policy.
CRS estimates that for the first half of FY2007, "DOD’s average monthly obligations for contracts and pay is running about $12 billion per month, well above the $8.7 billion in FY2006." Iraq alone is consuming an average $10 billion a month in 2007.
For long-term projections of the costs associated with the war, CRS relies on Congressional Budget Office numbers, which estimate that "additional war costs for the next 10 years could total about $472 billion if troop levels fall to 30,000 by 2010, or $919 billion if troop levels fall to 70,000 by about 2013. If these estimates are added to already appropriated amounts, total funding for Iraq and the GWOT could reach from about $980 billion to $1.4 trillion by 2017."
Here is a PDF of the full 45-page CRS report: CRSReport.pdf