From the Washington Post:

Many insurgents conducting attacks in Iraq are primarily motivated by money instead of ideology, [Lt. Gen. John R. Vines, the No. 2 U.S. officer in Iraq] said.
Citing interviews with captured detainees, the general said some simple attacks, such as placing a bomb or mine, are performed for as little as $100 or $150.
“So we believe that this insurgency is driven in large measure by money,” Vines said, suggesting the insurgency’s ideological supporters are funding the attacks. Some bombings cost hundreds of dollars or more, he said.