Amy - One place to start might be reviewing some reports from the Congressional Research Service (CRS); here are three recent (last few years) ones that might be helpful for you - "Costs of Major U.S. Wars" (June 2010), "The Cost of Iraq, Afghanistan, and Other Global War on Terror Operations Since 9/11" (September 2010) and "Defense: FY2011 Authorization and Appropriations" (November 2010). If you want to search CRS Reports further you can do so here: http://opencrs.com/.

Another resource that might help to qualify some of the data contained in the above reports is the National Priorities Project's Cost of War website. If you look on the "Notes and Sources" page there might be some additional spending information for you. Just an FYI, depending on if and what you are using the information from the National Priorities Project for: NPP is a nonpartisan, nonprofit organization, but it has been treated as a progressive, left-leaning one.