"If I can eat off the dollar menu at McDonald's (or Wendy's) twice a day:"

Yes, but you can easily cook your own french fries from potatoes costing under $0.20/lb. Even allowing cost for cooking oil and energy, your cost is likely to be under $0.25/lb. At Mickey D's, you will pay at least $0.99.

Similarly, you can easily make soft drinks like iced tea for far less than the $1 to $1.50/12 oz you will pay at Mickey D's.

Their hamburgers are a loss leader and tough to beat. But $2/lb hamburger goes to $0.50 for a quarter pound. Plus onions, bread, mayo or 1000 island, pickles. My guess is you can make a better hamburger than Mickey D's for $0.99.

So this is a matter of convenience rather than economics. And the variety of your diet is implied to be very limited. You can do lots of soups, stews, pasta, and others less expensively than Mickey D's dollar menu. And they are nutritious and better for you.