Cosmoveda is a hidden xberg gem. It's an Ayurvedic supermaket that happens to come with a kitchen with it. There you can find a delicious Ayurvedic lunch menu (with soup, main and a little dessert) in 3 different sizes, prices starting at just 5,90. Not to mention you can shop the ingredients that you just ate in case you want to try to cook Ayurvedic yourself.
The best cheeseburger in Berlin costs less than 5. Burgermeister is mandatory in town when you're craving for a basic and delicious to the bits chesseburger.
There's nothing more revigorating during the dark winter days than spending a couple of hours in a sauna. Germans have mastered the art of sauna and you can find amazing pieces of architecture in the public swimming pools (Stadtbad). I find Stadtbad Neukölln very peculiar in its neoclassic architecture and it's definitely a wallet-friendly option to enjoy some sauna time there. Be prepared to be naked in front of man and woman!