I agree with the posters who suggested vegetarian meals and more particularly, Homemade Soup day.
In fact, at least three times a month, our main meal consists of large bowls of tomato soup with grilled cheese sandwiches --- and of course, we've got dozens of cans of tomato soup, all purchase at 3/$1 or even less, a quarter a can. But we all really happen to genuinely like tomato soup.
On Homemade Vegetable Soup day, make sure everyone gets a big bowl of tasty soup with an inexpensive sandwich - maybe peanut butter, maybe a tomato sandwich sandwich, or perhaps eggsalad.
Once a month have macaroni and cheese.
If you throw together a cooked diced potato, a cooked sliced carrot, a couple cups of frozen vegetables, two cans of chili beans and about half a jar of spaghetti sauce -- then add water until soup consistency, you'll have a very basic Veggie Bean Soup to which you can make infinite variations, adding your family's favorite flavorings, almost any leftover vegetables or some noodles or some rice or some spice salsa - whatever's on sale, or whatever veggies are seasonal and thus low priced.
Go to your public library and explore the entire cookbook section, you'll find plenty of books that deal with thrifty nutritious meals.