From the St. Louis Federal Reserve newsletter, Liber8 Economic Information (May 2008):

It is difficult to measure the cost of the Iraq war and related expenses; it is at least as difficult to decide exactly what costs to measure. The May issue compares the two most widely cited estimates: one from the Congressional Budget Office and the other from researchers Joseph Stiglitz and Linda Bilmes. This month's newsletter also compares these estimates to U.S. GDP over the same time frame to get a better sense of the war's cost in relation to the entire U.S. economy.