I’ve been buying premium since 2010 when I bought a Mazdaspeed3 and most of my cars since have required it. Back then I usually paid $0.30-$0.50 more than regular. Recently it’s more around the lines of $0.60 more. Today I was in my wife’s car, thankfully it takes regular, and noticed the largest disparity I’ve ever seen. Went inside to buy a drink and asked the attendant and she said she’s never been asked about it before and wasn’t sure why it was so much more.
(If you can’t see on the pic, regular is $1.55 and premium is more than double at $3.19)