In our local 7-11 I can find the exact same carton of milk as sold in the supermarket, and the price is also exactly the same. So it depends. Some of the other stuff sold there is more expensive, but somewhat surprisingly it isn't that more expensive, so if I need butter or yoghurt or tea in a hurry then it's no big sacrifice to stop at the closer 7-11 instead of adding the extra five minutes (of walking) to go to the supermarket instead.