Here in Alberta, we pay about $0.10 per litre (Roughly $0.38 per US gallon) extra for 91 octane premium fuel.
As stated earlier, it's 100% gasoline ... not blended with ethanol.
It is my opinion that it's worth the extra dime per litre.
My 98 supercharged GTP requires it, but my 04 GP does not.
I burn it in both, regardless, and beleive that it pays for itself in better fuel mileage and decreased maintenance.
Ethanol is very dry and is extremely hard on rubber components (and fuel injectors).