There are a number of things to keep in mind if you want to figure out whether it's worth your while to make your own bread. Do you want to compare by weight? Or number of slices? Do you want 100% whole wheat, or will you settle for 50%? Or do you prefer white bread? Are you trying to avoid dairy products, or high-fructose corn syrup, or any of the numerous chemical treatments used in modern commercial breadmaking?
My starting point is that I like a hearty bread with over 50% whole-wheat flour. I avoid HFCS for calorie reasons and I avoid commercial breads with their weirdly spelled dough conditioner chemicals for hippie reasons. Thus, for my household's purposes, I'm not going to price-compare between my home-made bread and, say, dollar store white sandwich bread, even though the dollar store bread would "win." In our household, spending money on food with that little nutritional value is not a victory; it's a waste of money.
Breaking the prices down, I'll compare by slices, because in the end that's how I consume bread. I make a sandwich with two slices of bread, whether the bread is my heavy, home-made pain de campagne or a package of the lowest-priced, mass-produced bread I can find at the grocery store. A loaf can weigh 21, 24, 32, or 34 oz.; but keep in mind that the loaf I finish first won't be the lightest one -- it'll be the one with the fewest slices. And note that two supermarket loaves may cost the same but have a remarkable difference in the number of slices: family size versus sandwich size versus "country" size, even from the same manufacturer, can vary in price or weight, or maybe not, but they'll definitely vary in number of slices. And if you don't feel ripped off when you pay the same price for a family loaf as for a country loaf, but you get 5 fewer sandwiches out of the country loaf and don't feel any fuller at the end of your lunch, you probably should.
First, a quick summary of what goes into a loaf of my home-made bread, using prices from the affiliate links when given: 2 cups of whole-wheat flour; 1 cup of white flour; 1/4 cup wheat berries; 1 tsp salt; 1/2 tsp yeast, water. A 5-lb. sack of flour will yield about 19 cups. Whole-wheat flour is $4.99 per 5 lbs., or $4.99 per 19 cups, or $0.26 per cup, $0.52 for 2 cups. White flour is $1.99 per 5 lbs., or $1.99 per 19 cups, or $0.10 per cup. Wheat berries cost me $1.65/lb. last time I got them; 1/4 cup weighs about 2 oz. or 1/8 lb., so $0.21.
As for salt and yeast . . . I don't want to say the prices on those quantities are negligible, because I want to be rigorous here, so here goes. A 26-oz. canister of table salt costs $1.35, and Morton tells me that there are 491 quarter-teaspoons in the canister. That makes $0.0027 per quarter-teaspoon, or just over $0.01 per teaspoon. Yeast is a little trickier, but it's just algebra. I buy my yeast in 4-oz. (113.4 g) jars for $6.99. The manufacturer tells me that one of their 7-gram packets contains about 2 1/4 tsp, so I have a ratio of 2.25 tsp per 7 g. Now I have enough numbers: (2.25 tsp)/(7 g) = (x tsp)/(113.4 g), so x = 36.45 tsp per jar. Each jar sells for $6.99, or $0.19/tsp; so 1/2 tsp costs (and I'll round it up) $0.10.
Putting it all together, when I make a loaf of bread I use $0.62 for flour, $0.21 for "crunchies" (the wheat berries) and $0.11 for salt and yeast, plus water and cooking gas, which I'll ballpark at $0.25. My total is $1.19. I usually get a dozen slices of bread (plus heels) out of one loaf, so my price per slice is $0.10.
Thus, any bread I find at the store has to beat $0.10/slice, while also carrying the same nutritional kick as bread with 67% whole-grain flour, the added benefit of whole wheat berries, and the hippie holier-than-thou perk of using no high-fructose corn syrup or chemical additives.
That last loaf there competes in price to my own, so now I'm back to quality issues. At just 16 oz., that loaf must be pumped full of air so that the light-feeling slices don't taste too "disagreeably" like whole grain. That bread will also be soft and squishy, without the sturdy grain and hearty mouthfeel that I prefer. But, you know, if you like a bread that melts in your mouth without your having to actually chew it, and if your household does not find dough additives and HFCS objectionable, then maybe you would prefer to buy this bread than make your own.
Which brings me to Ray's question from the other day about calculating time. It's a complicated question, and one I will be addressing sometime soon. Here's one consideration. My home-made bread requires 5 minutes to mix, and another 5 minutes or less (in short segments) to turn out of the bowl, shape into a boule, place into the proofing bowl, place into the oven, and retrieve from the oven. (The fermenting and baking times are hours when I am asleep, or I can work on other things, or I don't even have to be at home.) On the other hand, it's a 20-minute round trip for me to travel to my neighborhood supermarket for its generic brand whole-wheat bread. Please consider: not only do I subjectively think that last loaf of bread is terrible food, but actually it takes twice as much time to obtain than does my own bread.