I get what your saying about things like multi packs of crisps or doughnuts being cheaper than the good stuff but I don’t think healthier food is always dearer in comparison.
You don’t need to not eat!
For breakfast what about
Weetabix
Porridge
Scrambled egg
All pretty cheap, filling and healthier choices
Lunches
Soup ( you could make some once a week and have a few days/all week)
Egg ‘muffins’
A tuna sandwich ( if you mix the tin of tuna with mayo and a bit of sweet corn it would do 2 days sandwich)
Again all pretty cheap, will fill you up
Dinner
Big pot of bean chilli (have it with jacket potato one night, rice another and you could have a portion on its own for lunch as well)
Basic spaghetti with a sauce ( tin of chopped tomatoes, onion, garlic, bit of purée simmer) could add in any veg you have knocking around
Veg pasta bake ( use courgette, mushrooms, chopped tomatoes)
Jacket potato with beans/tuna
Omelette
Lentil stew
Again all fairly cheap and will fill you up
Snacks
Carrot sticks
Bananas
Cucumber sticks
Jelly ( fairly low cal, cheap to buy and takes minutes to make, let it set overnight)
Some fruit and veg is expensive Yes but things like cucumber, lettuce, bananas, carrots, cabbage, broccoli are very reasonably priced
Nothing wrong with buying what you describe the as “Fatty” meat that will fill you up more than the lean imo and it’s not bad for you when you think of the size portion you actually eat
Same with beans you don’t need the low sugar ones, the standard ones are absolutely fine and if your really worried about the sugar just have half the tin.
Thing the key thing is probably planning wisely