Unless you have a 500 CFM carb laying there ready to install it would take you about a zillion miles to recoup the cost of a new carb with the minimal mpg gain you would get.

As others have said, getting everything else in the engine tuned and tweaked for mileage and adjusting your driving habits would show you a better return on the money invested.

Your biggest return on the dollar would be to ride the bus to work.......