On the other hand, I'm familiar with a truism that food on the edges of a supermarket -- the produce and dairy sections -- tends to be healthier and cheaper than the stuff in the aisles -- the breakfast cereals and canned or jarred goods and funny drinks and other processed foods. So when people in my forum started agreeing with each other that one could live on fast food more cheaply and more conveniently ("Unless you counted in time to drive to and from the fast-food joint", as if that were somehow not worth counting) than on balanced meals at home, this made no sense to me and I started arguing otherwise. I didn't seem to change anyone's minds, but I'll present my argument here anyway.
So, how cheaply *can* one eat out? I assume one needs to get 2000 calories. I also assumed 1000 calories per Whopper or Big Mac, at $2.50, or 400 calories per dollar burger, or 400 calories per dollar bagel at other places. These all give $5/day, or $155 in a 31-day month. Possibly you can find lower, or perhaps combo meals with large soft drinks give more calories (as pure corn syrup) for less money. I haven't pursued more accurate numbers yet, and no one actually challenged my numbers.
For the eating at home side, a sample budget:
$2 for 6 glasses orange juice concentrate; $10 month.
$2 for 5 servings of carrots; $12 month.
$2.50 a loaf, half loaf a day; $37.50 month.
$3 peanut butter, 1/4 jar per day; $24 month
$4/lb ham, 1/4 pound day; $32 month
$3 16 cups milk, 4 cups a day; $24 month
$2/lb green vegetables, 1/4 pound a day; $16
I did not, in fact, tune this budget to meet a total (though in the original discussion I'd said $150/month for fast food, but more months are 31 days, and the healthy budget numbers fit a 31 day month more closely.) The budget isn't tuned at all, but is my first draft at something which seemed reasonably healthy and cheap. I went for orange juice and carrots for vitamins A and C, half a loaf of bread because that I think that's how much I'd use if I lived on sandwiches, ditto for the peanut butter and ham. 4 cups of milk a day is a bit high -- I grew up on 3 -- but I wanted to make sure there was plenty of calcium. (And I may have rounded up since 4 was easier to calculate with than 3.) The vegetables are a bit handwaved; I get broccoli for that price, but pre-washed spinach is $1.99 for a 9 ounce bag (which, though, I tend to use in 3 or 4 days, though the bag says "2 servings".)
At any rate, I figured out those basic needs, drew upon standard prices -- no sales or anything -- and got that total. For yet more coincidence, $155 is almost exactly my average groceries expenditure per month over the past two years. More on that later.
Other notes: I was assuming $2 a can for concentrate producing 48 ounces of juice; I'm not sure what the real price is. The carrots in mind are a pound of peeled cut or baby carrots. Organic ones, in fact; I think you can get normal ones for $1.50/pound. The diet is monotonous, but no one specified variety, and there have been phases of my life where switching between peanut butter sandwiches and ham sandwiches would have been an improvement. Bread and peanut butter are a rather expensive version of "grain and legumes", compared with rice and beans, but I was trying to compete on convenience. A rice and beans diet, at $1/pound for rice or beans would shave $38.50 off the budget, and another $24 if the ham were replaced by beans. On the other hand, I'd have to throw in some butter or oil and condiments, to make the food more palatable; peanut butter at least brings its own fat and salt.
How healthy is this diet? 100% US RDA vitamin C from the juice, 270% vitamin A from the carrots, plus possibly more from the milk, and more of both from the vegetables. I need to double-check the rest of the numbers, but I think 120% calcium; 64 grams of protein not counting the bread, which could add another 16-32 grams; 2100 calories, or more if the milk isn't skim; 42% of fiber not counting the bread, the contribution of which could vary a lot.
Not perfect, but much better than the fast food diets with close to zero vitamin A, C, calcium, or fiber (well, the bagels could have lots of fiber.) And it's sandwiches, drinks, and vegetables which need at most a minute of microwaving in preparation; you could be done with your meal before someone had finished driving to a fast food joint.
How does this made up budget compare to the real world? As I said, I seem to spend $155 on average. On the other hand, I've spent $72 a month on eating out. That's probably not more than 14 meals (out of 93) eaten out a month, and probably less, but it's not a trivial amount of calories, implying I'd be spending more than my budget if I lived entirely on groceries. But, looking at how I eat, this makes sense: I drink non-from-concentrate orange juice, and more of it than 8 ounces a day. I eat a fair amount of nuts and cheese, especially nuts, and those are expensive. I eat more fruit: bananas and apples and berries, among others. I'll buy sushi from my supermarket and count that under "groceries" because it came from the supermarket. I use a fair bit of olive oil. So it's probably just as well that I don't drink that much milk, and often eat pasta or even rice instead of bread, to bring the budget back down. Still, there's convergence; I spend more, because I eat better than my budget. But the budget stands on its own.
Cheap improvement: a banana a day would add $6 to the budget, which could be paid for by drinking one fewer glasses of milk.