In my experience with baking bread, buying from the supermarket is cheaper just because of the price of flour and other ingredients (however, even if I'm not as consistent as a good professional baker, my bread is always better than that I can find at the supermarket).

From an energy point of view, well, probably buying is also better, because cooking lots of bread at the same time reduces the amount waste per serving (although in the winter that waste heat is surely appreciated :D ); here a bread machine is going to be better than cooking in the oven like I do.