I am paying 91 cents per liter for 90.5 (US) Octane midrange, the lowest grade sold here on base. The locals pay 1.60 Euros for same (we are allowed to pay US taxes on fuel they must pay German taxes hence the difference) so approx 6 USD per gallon of midrange.
The price difference for premium is huge on base, even though the Octane difference is not. Premium which is 98 Euro Octane = 93.5 US Octane costs approx an extra 91 cents per gallon.