In 2007 to 2015, DOD spent $58.6 million for 2.0 million gallons of green fuels, or $28.79 per gallon. These figures come with 2 caveats: (1) DOD purchased far more petroleum than green fuels, and bulk purchases typically reduce prices. (2) Green fuels often contain less energy per gallon than petroleum, and thus, costs per gallon can understate the relative costs of green fuels.