I California, we pay a minimum of .20 per gallon for 91 octane. I have not seen anything higher than 91 here, but it's possible it may be at a few special stations.
More than likely, the price for premium is .25 more, but I usually buy at Costco because this is the cheapest.
My wife drives 25 miles one way to work, so we will be spending more than $100 a year extra on premium. I'm hoping she wont have to fill the tank more than once a week.