Rice and beans are a complete diet. Potatoes and milk are a complete diet. Frozen vegetables fried with spices are tasty, nutritious and very fast to prepare. Porridge, aka oats with milk or water is also dirt cheap and nutritious.

I spent an unfortunate length of time watching my pennies when it came to my foodbudget, in a western country. Being poor sucks but healthy food isn't expensive unless you mean prepared, ready to eat expensive food.