There are differing prices per gallon depending what vehicle it is used for… but as I can tell, in 2013 the price of a regular gallon of gas for an Air Force jet was under $4, and a gallon of “Green” fuel was $59.

✦ Military Signs Contract for Green Jet Fuel That’s Nearly 16 Times the Price of Conventional Fuel
✦ Obama, Clinton Foundation Donors Sold ‘Green’ Fuel to Military for $149 per Gallon
✦ As Pentagon invests in green fuel, critics focus on the cost
✦ Military Green: U.S. Air Force Flies on Biofuel
✦ US Navy Green Fleet Makes Biofuels the New Normal