Iraq is atop 5% of the world's oil, and about one trillion cubic feet of natural gas, this without any exploration program for 35 years. It is estimated that Iraq's extraction technology is so antiquated that an upgrade (now in progress) to more modern tech will yield a 30% productivity gain, when compared with the pre-war extraction rate. Thirty percent of five percent may not sound like much, but with Iraq's dire need to pump oil for it's nascent economy, OPEC will be greatly debilitated when Iraqi crude comes fully online.
Our country uses 140 billion gallons of petroleum-derived fuel per year, so a mere 20 cent reduction in the price at the pump, by virtue of the added supply, would yield $28 billion per year to the American economy to help pay back the cost of the war. This is not to mention the cost of other petroleum derivatives, like plastic products.
Wars are fought on both strategic and psychological battlefields. The jihadis fully expected that we would come to Afghanistan, where they expected to defeat us there as they had the Russians. They surely never expected us to take out Saddam, and our presence in their midst has dealt them a severe psychological blow, such that they have been impotent to attack us again on our own soil. Can we attach an "opportunity cost" to what HASN'T happened, by virtue of the Iraq war?
This war has made Iran and China more powerful.
Talabani has signed agreements to buy weapons and cooperate economically with China.