Not just cost of food but time to prepare foods. Many really don't have the time to make a better for you less processed dinner even if the same price (I learned how to make cheap, good foods, but they were time consuming).
Cooking healthy, inexpensive meals is mostly a matter of education.
I own a number of cookbooks where every recipe has 5 ingredients or less.
A big trick to eating healthy without much fuss is to cook food in bulk. Cook a large package of chicken, then freeze in portions for up to a year! There are 1,000 EASY, cheap ways to flavor plain poultry + cooked starches + cooked vegetables. Any combo of meat/veggies/starch + water and bouillon cubes= soup. Eggs are versatile, inexpensive, and easy to cook.
One easy cheat is to save the packets that come with fast food, like honey mustard sauce. ANOTHER easy cheat is to double a fast food meal. One order of fried rice is easily doubled by adding frozen veggies + scrambled egg(s). Soups are also super easy to double. Any leftover food sauce- like Asian brown sauce can be saved and used to flavor a 2nd round of veggies. Many sauces can be doubled just by adding water; it often healthier and tastes better.
Black bean soup is a favorite recipe: super easy, inexpensive, and super variable recipe. Just about every ingredient, even the black beans, is optional: sub just about any bean, except green/pole beans. Use any carrots: baby, frozen, canned, fresh chopped carrots, or no carrots. I love pre-chopped frozen onions!
1. Sauté chopped onions and carrots in a pan with any type of oil until they start to brown.
2. Add 2 cans of rinsed (black) beans, 1/4 cup orange juice (or not), 1/2teaspoon cumin (or not), garlic (or not).
3. Add 2 cups of any type of broth, OR 2 bouillon cubes+2 cups of water, or even plain water! (w/salt at end), maybe 1/8 cup wine/mirin/miso
4. Simmer for 30minutes or longer. Longer tastes better.
6. Just before serving add salt and pepper to taste. Maybe add: hot sauce, peppers, chilies, spinach, corn, etc.
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Serve plain OR with chicken, chips, crackers, jarred salsa, sour cream, plain Greek yogurt, guacamole, chopped mango, and/or:
Salsa A: Mango, cilantro, chopped red onion (raw or cooked)
Salsa B: Avocado, cilantro, chopped red onion
Recipe can be doubled, stored in fridge, and/or frozen. Anyone can make this, and it is even vegan!
As with much of the debate in this thread, it is mostly just a matter of making excuses. Every library has recipe books.