The ones who make out like bandits on the deal are the credit card processors. They're not just taking a percentage, they're taking it off the top. I don't remember the exact numbers, but the last time there was a big jump in gasoline prices, NPR (IIRC) did a piece on it and ran the numbers, and they figured that the average small gas station's owners were actually losing money on gas sales after the credit card processors took their vig.Doing the math from @SlugSlinger, all of these places are running <10% over their cost. In Sam's case, <4%...they sell so much volume is other stuff, the gas is lost in the numbers.
SB aren't making sh*t except to cover cost to provide gas...look at what it would cost if they actually put a margin that made them money, say 15-25%. The gas is bait to consumers inside to buy stuff that they can make money on.