Milk and potatoes can give a good base of vitamins and minerals.
Potatoes are pretty cheap and very easy to grow if you have the time and will to try it. Just toss a few potato halves into a bin of dirt, water periodically and you'll have more potatoes than you know what to do with
Toast can be a fairly cheap breakfast, although not very filling. It's easy to quickly eat as you run out the door too
I've found making sure your dinners have multiple dishes actually makes the food go further and helps in saving money on groceries overall compared to not
A bag of freezer veggies can keep in the fridge for almost a week pretty easily, and it's very easy to pour a bit out, nuke it in the microwave for 30-60 seconds and help round out your meal.
Hotdogs cook very well with ramen noodles (you can also sprinkle in some frozen corn too!), and that can make 2-3 meals for a single person
if you're in the states, Aldi is genuinely a really good option to save money on groceries, plus their store brand stuff usually has less sugar than name brand
white rice is usually dirt cheap and a good base source of nutrients