My DS is very fussy, for genuine reasons due to ADHD (possible ASD as well but that’s not diagnosed at the moment).
He is totally wise to any kind of disguised vegetables as ND kids usually are - can detect them a mile off.
Are there any vegetables yours will eat?
DS will eat carrot sticks and sweet potatoes (also eats actual potatoes). Will eat some salad leaves or spinach leaves if completely plain. So he gets carrot sticks with every meal regardless of what it is. And sweet potatoes are eaten a lot (he likes them very well done).
Then I’ll often serve the fruits he likes with the meal or for pudding - bananas, pear, satsumas, maybe apple. Eats strawberries and mango too but I know you said not expensive.
Meals he likes are a mild chicken curry, cottage pie (very plain, just mince and garlic and maybe a bit of chilli in a stock cube for the base, potato topping - carrot sticks on the side), will eat lasagne if not too tomatoey, stir fries based on chicken and noodles.
For very budget macaroni cheese is good - and again I put carrot sticks on the side!
Edit - the best part is, DD16 (not fussy at all) is vegetarian and I am pescatarian 😂