There's no way to account for the true costs of war. These include not only the direct expenditures on munitions, the lost production of useful things while soldiers are deployed killing, and the permanent lost output of those who never return, but also the emotional costs of worry and disruption borne by those who serve and those who stay behind.
Some direct costs can be measured, albeit without much precision. The costs of U.S. wars per capita have ranged from a low of $27 for the Gulf War (our allies picked up all but $7 billion of the $61 billion total) to $15,655 per head to wrap up World War II. We've yet to tally the bill for our current campaign against terrorism, but it's estimated to be costing $1.2 billion a month more than the military's peacetime budget.
Graph: Per Capita Cost of War in 1990 Dollars