I agree that the difference in the price of gas wouldn't impact my decision, but if you're concerned about it just remember that (likely) you'll get 3 tunes with your tuner. You can fill with 93 octane one time, and flash the 93 tune into the car; drive it and see if you like it. Then, although you'll still have the 93 in the tank, flash the 91 tune into the car and compare. If there is not much of a noticeable difference in your opinion, run out the 93 and on the next fill-up fill it with 91.
Eitherway, the difference that you experiece over the stock tune will be amazing, even when running 87 octane fuel!