Turns out they lied about that a little bit. They didn't take into account the insurgency that happened because they mismanaged the war from it's inception. Instead of listening to Eric Shinseki, they fired him. Good job, that. Turns out he was right and they did need more troops. (Although staying the hell out of there would have been a better option.) Instead of oil flowing and paying for the war and the post-war reconstruction, Iraq is producing 1/3 fewer barrels of oil than they did before the US invasion. That is because the oil infrastructure is attacked daily by the insurgents.
This costs you money. Even though most Iraqi oil does not end up here on the US market, it is still the second-largest active petroleum reserve in the world. When Iraqi production is compromised, the oil market tightens and the price of oil per barrel goes up. When this happens Americans pay more for gasoline, more for heating oil, more for energy across the board.