William Nordhaus, a Yale economist who published perhaps the most extensive independent estimate of the potential costs before the war began, suggested a war and occupation could cost anywhere from $100 billion to $1.9 trillion in 2002 dollars, depending on the difficulty of the conflict, the length of occupation and the impact on oil costs.

The most current estimates of the war's cost generally start with figures from the non-partisan Congressional Budget Office, which as of January 2006 counted $323 billion in expenditures for the war on terrorism, including military action in Iraq and Afghanistan. Just this week the House approved another $68 billion for military operations in Iraq and Afghanistan, which would bring the total allocated to date to about $400 billion. The Pentagon is spending about $6 billion a month on the war in Iraq, or about $200 million a day, according to the CBO. That is about the same as the gross domestic product of Nigeria.

Joseph Stiglitz, a Nobel Prize-winning economist and self-described opponent of the war, puts the final figure at a staggering $1 trillion to $2 trillion, including $500 billion for the war and occupation and up to $300 billion in future health care costs for wounded troops. Additional costs include a negative impact from the rising cost of oil and added interest on the national debt.

In the buildup to any war, financial costs rarely play a big role in the debate, especially for a superpower like the United States, which is presumed to have virtually limitless resources. But economists like Wallsten and Davis say there is no reason wars cannot be subjected to the same type of cost-benefit analysis as other government activities.