We have a limited allocation of discounted hotel rooms in the vicinity of the conference venue. These rooms can only be booked when registering for the conference. There are also less expensive options available. When searching for accommodation on portals like Kayak, try to search in the vicinity of Bahnhof Dammtor, which is the railway station just across from the venue.
Also consider hotel alternatives like Airbnb, Wimdu or 9flats. These are not necessarily cheaper, but can sometimes host more people. TripAdvisor sometimes lists (real) B&Bs that are not listed in other portals.