Don't bother with it. The cost difference you'll pay is more than offset by the poorer mileage you'll get. I could get all technical, but suffice to say that one gallon of ethanol contains just about half as much energy as a gallon of gasoline. Your car still needs the same amount of energy to function, so you'll use more ethanol. Most people see a 30-40% drop in fuel economy when running e85. Paying $1 less per gallon won't offset this.
Also, the alcohol in ethanol isn't terribly good for your rubber fuel lines. I'd just stay away personally.