Mensa is a food court for student. And as it is for student, the price is surely cheap. The main menu in Mensa is called ‘Essen 1’ and ‘Essen 2’.
Essen 2 costs €1 and Essen 1 costs €2. The cost is covering the carbohydrate part (rice or noodle or potato) and protein part. Usually Essen 2 is a vegetarian menu, but sometime they switched. For Essen 1, the cost will also cover a dessert (yoghurt or fruit or instant pudding) and/or soup.
Other than these main menus, there are also other corners like Salad corner, Pasta corner, Vegetarian corner, Burger corner, etc. But these corners are a bit expensive. So if you in a tight budget, the main menu will be the perfect choice.