Do you pay extra for "branded" gas from companies like Exxon, Mobil, or BP? IMHO – Gas from any reputable company is gas.
The other day, I paid $3.61 per gallon at my local BJ's Club. On the way home, I saw prices as high as $3.84. In my area, the Citgo consistently has the lowest prices and the Shell has the highest. At some point, one would expect the Shell customers to switch to Citgo; however, they do not.
Why not? Despite all the protestations about gas prices, I think most people are lazy and not sensitive to gas prices. They just fill up in the most convenient spot without even considering the cheaper gas one block away.