I have quite a lot of experience with this. but it was western Europe level of poverty, not serbian. i could always look at my peers ordering food 2-3 times a week and feel quite comfortable in my food costs.
The main advice is: shoplift. This is a parody btw. Totally not my genuine advice that I apply on a daily basis.
Then it's also important to get very acquainted with the options you have. Much of my advice simply won't apply to the Serbian market.
I go to several supermarkets (without a car) to assure both highest quality and lowest cost. The next is quantity!! Get large amounts of things, it is always cheaper. Sometimes online options are good in this case too, especially if they suspend shipping costs over a certain amount.
A few examples:
After years I found out a certain brand of grape juice was a lot cheaper (and sweeter) than OJ, and have only been consuming that since.
Zucchini are a great fruit option to make in large batches.
Dates are great. cheap, they don't go bad fast. The tunisian deglet nours and iranian mazafati dateshave a great cost-digestibility balance
Skim milk powder pancakes give me a filling breakfast everyday and are fairly cheap.
Master broth for protein. check out my signature. recently i've been shoplifting calf feet because the butcher near me is inside a supermarket and i just pretend it's a bag from a different store when i go by the cash register with all my other groceries that i do pay for but it's still not expensive even when you pay for it. This entire last sentence was a total parody and not true. never happened.
Just be flexible, learn a lot, and try to make choices that result in the best possible option. This seems super obvious but it's good to remind yourself you can still find out a lot.