most of the decrease in gas mileage when lifting comes from the heavier wheel/tire assembly. if you lift and keep the same tires the decrease you see will be due to being less aerodynamic. im not too sure on numbers, but would think it would be not too devastating.
i had a 3.0 auto with 3.73 gearing with spindles and junk lifting it up. with 33 inch bfg mud terrains i felt extremely lucky if i ever hit 15 mpg, regearing would have helped out significantly though.