Eat unprocessed foods, mostly plants, not too much...
Don't eat anything your greatgrandmother wouldn't recognise as food.
Never be overweight.
Why? Because it will help you to stay OUT of the hospital. And we all know that's expensive...
Of your 2000 kcal of you day, eat 800 kcal for breakfast, 600 for lunch, 500 for dinner and 100 for snacks. Don't skip meals, that's bad for your blood sugar and thus for you overall health. I wouldn't follow the tips above about skipping breakfast or fasting and going for the free meal at lunch... If you want to be that cheap: take an extra bag and sneak the free food in so you can eat from it during the next day at breakfast.
Your main meal should be breakfast, not dinner!
Always prepare your food at home and take it with you.
Only eat meat/fish two times a week. Red meat only max once. Buy it when it's on offer and store it in the fridge. Always buy on offer. Like other elements of frugal existence, you get used to a differerent eating pattern.
What to eat (always whole grain):
Breakfast: rolled oats with nuts and fruits. Or bread with egg and fruits (for weekends).
Lunch: bread with (homemade) hummus or (homemade) nutpaste (buy nuts, grind them, and tata: paste (no additives and extremely healthy, especially walnuts) with fruit or have a salad (that is: eat carrots, selery, cucumber etc.)
Dinner: beans with loads of veggies. Only little potatoe or pasta (60 gr or 2 ounces for 2 persons).
Snack: fruits or veggies (like carrots, selery, cucumber etc.)
How to make cheap dirtcheap: grow the veggies (and especially fruit) yourself.