The canteen food in Aachen is being subsidized by the state. The student union “Studentenwerk” which runs the Mensa is not allowed to generate income. This means that in comparison to many other countries you can get relatively good food for a relatively low price:

Main Meal                          2,60€
Vegetarian Meal               2,10€
Stew                                    1,80€

There are many different buildings, but the menu is almost identical. You can find all the buildings on this website:
In some mensa buildings, like for example the new Mensa Academica, you cannot pay with cash anymore. You need to use one of the credit machines at the entrance to load a credit onto your bluecard.