From the Summary and Introduction: "Estimates of the total cost of a military conflict with Iraq and the conflict's aftermath are highly uncertain and depend on many unknown factors including the actual force size deployed, the duration of the conflict, the strategy employed, the number of casualties, the equipment lost, and the need for reconstruction of Iraq's infrastructure. Of the many force-level options being discussed, CBO [Congressional Budget Office] examined two representative examples. Both alternatives are based to some extent on the forces that the Department of Defense (DoD) had previously indicated it would require for a major theater war. Using those forces, CBO employed various methods to develop its estimates. As appropriate, it used cost data from prior and current military operations--most notably, the operations in the Balkans, Afghanistan, and Desert Shield/Desert Storm. In addition, CBO estimated costs using categories similar to those in DoD's annual reports on the Overseas Contingency Operations Transfer Fund--personnel and personnel support, operations support, and transportation. Under the assumptions incorporated in its examples, CBO estimates that the incremental costs of deploying a force to the Persian Gulf would be between $9 billion and $13 billion."
Congressional Budget Office: https://www.cbo.gov/