You have to be kidding me... you cook 2 or 3 times a week... as in 3 meals a week or you meal prep 3 times a week. Fucking hell...
780E a month... Which city you living in you fuck up? Go to Aldi or Lidl. Check Rewe as well, this morning Rewe have Hackfleisch Gemischt for 2.22E a half kg and chicken breast for 6.99E a kg and pork tenderloin for 4E a kg.
Buy oats, rice and pasta in bulk, all super cheap from Aldi and if you buy Ja! from Rewe. Buy potatoes, 2E for 2.5kg. So many cheap carbs sources.
Protein buy red kidney beans, under 30 cents a can from both Aldi and Rewe, look for meat on offer in supermarket adverts and buy lots at a time, chickpeas and lentils are cheap as well, not as cheap as beans but not bad. If you have an Asian supermarket in your city go buy them in 2kg bags from the indian brands. Buy milk.
Cook. Everyday. Take you food for lunch. My Mensa costs anywhere from 3E to 6E a day for lunch. For 30E you can buy all your weeks food. Buy tupperware from amazon, just one. Cook in the evening and make twice as much. Eat half, other half in the tupperware. I have 6 of this one: amazon.de/LOCK-LLG445A-Frischhaltebox-Boroseal-rechteckig/dp/B00G1RI350/ref=sr_1_3?ie=UTF8&qid=1529163658&sr=8-3&keywords=glass storage containers 1 litre
Don't buy peanut butter in the stores, it is overpriced and other than the Bio one from Rewe they all have too much sugar. Get it from myProtein.de along with a 5kg bag of protein when they have their 50% sales.
Aldi do frozen vegetable bags for a euro a kg.
Don't talk shit about spending so much if you won't even try. Sure I spent 50E today for the week. But I also have more income than you. If I didn't go to Rewe, didn't buy avocados, stuck to white potatoes instead of sweet I would save more than 15E on that. My sweet potatoes were the second most expensive part of my shopping after meat because it is 2,50 a Kg vs white potatoes at 2E/2,5Kg.
End of the day, it's your choice.