I suspect it's quite common. Gas is an ideal loss leader -- the price on the sign, and your need for gas, gets you into the station. But now you're there, and you're less sensitive to the price of a Coke or bag of jerky.

But there are a lot of markets where price competition drives down the margins for the "main" product, so that the retailers make up their profits on smaller stuff. I've read that a store selling electric guitars or bicycles makes relatively little profit on those items, but instead, makes their nut on accessories, things that wear out, service, lessons, etc.