What to eat?

One meal in Mensa costs around 2,6-2,8 euro (meat component + potatoes/french fries + salad). Mensas are usually open for lunch, from Monday to Friday between 11:30-14:15. Venusberg Mensa is also open on Saturday 12:00-14:00. For details on prizes and locations of Mensa in Bonn see the Studentenwerk website.

There are a few places that are worth visiting when it comes to food, but usually prizes range between 2 and 10 euros. 

On daily bases you would rather look for a grocery shops than a fancy restaurant. In Germany the cheapest ones are: Aldi, Lidl, Netto and Penny, but keep in mind that they may not have the articles that you need, so you may also want to visit Rewe. The prizes there are reasonable and the assortment is much bigger. In Venusberg there is an expensive supermarket: Kaisers, whereas near Poppelsdorfer Platz  (nearby you will have the first three modules) you may find Rewe and Netto.