Seems like the differential here grows when the price of regular drops...I suspect they are trying to attract customers by posting the low regular price in huge numbers and then socking it to those who have to buy midgrade or premium, they are probably less likely to drive off after they've gotten out of their car to read the tiny numbers for those on the pump. The differential can easily be 70 cents, don't think I've seen it be less than 40 cents in quite some time. Seems to me that stations that are more rural and/or off the beaten path tend to have a smaller differential...
I can't remember the details for sure, but I think I drove off from a local Shell station when they had an 80 cent differential...there was a Mobil nearby that was not so crazy, but most of their pumps were not working for some reason. Ended up at Crazy Ed's Cheep Gazz N Booze and putting some Techron in the car later... ;^)