Hybrids less important than Hummers

Assuming the same # of miles will be driven, is it better to spend $X to increase an individual vehicle's mileage from 10 to 20 miles per gallon, 20 to 30 mpg, 20 to 40, 20 to 50, 20 to 100, or 20 to 1000?

Space for you to ponder in...

( Answer! )

(Hey, mrs_feltner, behold the power of MATH!)