I think nutritionists now agree the basis of any nutritious diet is adequate protein together with plenty of vegetables. Data shows people who focus on protein, healthy fat and vegetables feel full sooner and eat less than others.
My basics, from Aldi and Tesco, are chicken, canned fish, eggs (from my wife's hens), cheese, cottage cheese, butter, plain and Greek style yogurt, a lot of onions and cabbage, celery, peppers, carrots, swede, squash, beetroot, canned tomatoes and passata, canned/dried beans, oats, barley, together with olive oil (used sparingly) and, from time to time, courgettes and aubergines. None of these are very expensive. My wife is a bit more extravagant!
Dr David Unwin asked his patients about costs. They told him any increase in the cost of a healthy diet was offset by savings on junk food, sweets and drinks.