First off you will probably not gain any gas mileage. Bigger rolling resistance and the height, it's like you are pushing a box down the highway.
As far as cost, it will vary but probably would be in the 1200 dollar range for both axles. Say 300 a piece for gears and install kits and 300 each axle for labor, depending on where you go for service.
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Wheel it like ya stole it!