A conservative estimate of the cost in dollars alone to the US of the Iraq war (ie not counting lives, pain, political or strategic or opportunity costs) is $800 billion to date. That’s the amount appropriated by Congress. It doesn’t count the so-called ‘black budget’ and it doesn’t attempt to count the foreseeable future costs– taking care of our wounded, for example.
The population of Iraq is about 32,000,000. So that means the war cost us about $25,000 per Iraqi.
I think my suggestion back in 2003 that instead of staying in Iraq we just give every Iraqi $3000 per year for the next year or two is looking awfully good in retrospect.
Note that Iraq GDP per capita in 2010 was $3,800 under a purchasing-power-parity measure, but valued at the official exchange rate was only $2,567. So basically for what we spent on Iraq since 2003, we could have given every Iraqi the equivalent of their share of GDP every year until now, and it would have cost about the same as what we spent. And there wouldn’t be the killed and wounded.
Perhaps buying countries is actually cheaper then invading them. Note, however, that the money would have to actually go to the people, not to the government or the military or the exercise would be fairly pointless (see, e.g., Egypt, Pakistan).