I live in a WG (a group rents a flat or a house and everyone gets a room, common kitchen and bathroom) and spend around 600€/month including health insurance (you need that, 70€/month, don't know about foreigners). Depending on the city, it's harder or easier to live on that because rent can differ a lot. (200-400€/room, I pay 250€.)

edit: To clarify - the university requires you to have health insurance, but there is a special tariff for students. Students below 25 are mostly covered by their parent's insurance here, after that you need to cover yourself.