Berlin is famous for its ubiquitous fast food, especially Döner kebap places you can find at any corner. But there are also a lot of cheap pizza places – and here is a map with them: Where to get big, round (Ø 26cm) pizzas for 3.50 EUR max.
Thanks to the suggestions of the fine people on the Facebook group Free Advice Berlin, I could extend the six places I knew to currently 24 on the map. Thanks again! 🙂