If you're willing to eat the very cheapest bread available at the store (the store-brand of Wonder Bread, likely), it's probably least expensive to buy bread. If the idea of having your bread adhere to the roof of your mouth makes you a bit ill, however (like me!), you may be able to save money by making your own bread.
The bread I buy at the grocery store (whole wheat, minimal extra filler ingredients) costs, on sale, about $2.50 - $3.00 per loaf. After a while, this started to seem like a place I could save some money, have some healthier bread, and make my homemade-bread loving husband happier.
This is the recipe I use, which I calculate costs me about $0.70 per loaf (not counting the energy it costs to run the oven). I also think it might be a bit cheaper for you depending on where you live, since I know flour and sugar were cheaper in Texas than they are for me now in Boston.
I've also updated the recipe to use instant yeast, which I really like because it shaves a good hour off your time since you don't need to proof the yeast or let it rise before you shape it.
Whole Wheat Bread (recipe adapted from Mel's Kitchen Cafe)
2 3/4 cups warm water (for all intents and purposes, free)
1/4 cup sugar ($0.05)
1/4 cup oil ($0.06)
1 tablespoon instant yeast ($0.04)
1 tablespoon salt ($0.01)
1 tablespoon vital wheat gluten ($0.08)
1 tablespoon nonfat dry milk ($0.10)
1 cup white flour ($0.17)
4-5 cups white wheat flour ($0.89)
Combine yeast sugar, oil, salt, gluten and dry milk together in the bowl and mix with an electric mixer or by hand. Add white flour and mix well. Continue adding the whole wheat flour until the dough cleans the sides of the bowl and the dough is soft but not overly sticky. Knead for 10 minutes until a soft, smooth dough has formed.
Form into 2 loaves. Place the loaves into lightly greased bread pans and cover. Let them rise until the dough has risen about 1 1/2 inches above the top of the bread pan.
Place the bread pans in cold oven. Turn the oven on to 350 degrees and bake for 38 minutes.
Remove from the oven and turn out the bread onto a wire rack. Let cool completely before slicing.
I slice up my entire loaves, put them in bread bags (which I've saved from my store-bought bread), and stick them in the freezer. Easy and delicious.