1. Iraq has oil - a lot of oil.

Iraq: Oil and Economy

Iraq's Oil Reserves: Untapped Potential
While its proven oil reserves of 112 billion barrels ranks Iraq second in the work behind Saudi Arabia, EIA estimates that up to 90-percent of the county remains unexplored due to years of wars and sanctions. Unexplored regions of Iraq could yield an additional 100 billion barrels. Iraq's oil production costs are among the lowest in the world. However, only about 2,000 wells have been drilled in Iraq, compared to about 1 million wells in Texas alone.

That's only a few years. But consider where gas prices were a few years ago. That's a good first approximation of the difference that would have been possible without sanctions and two insane wars. There are all kinds of maybes hanging off that - e.g. an Iraq/Iran war could have pushed prices up even further, without US involvement. But assuming relatively stability, I'd guess that putting Iraqi oil on the market at full extraction levels would slash the price by at least a third.

2. The Iran factor. I'm not sure if people are buying just in case, or not buying because it makes no difference. But prices go up every time sabres are rattled. Without Iraq, there would be no credible threat to Iran, so we could lose maybe 20% off the current price just from the Iran factor - which is being played up by both parties, because Iran always profits when prices increase.

3. Speculation is the great unknown. There's as much evidence for political manipulation as there is for profiteering, but I don't think the difference is going to be more than another 20% or so.