Don't forget to consider the two (Accor) Novotel properties, one near the Tiergarten and the other on the southeastern edge of Mitte. Definitely on the business-traveler side, but good value for the money.
They are showing rates as low as Eur. 89,- and Eur. 79,- for your dates, respectively:
http://www.novotel.com/novotel/fiche...he_hotel.shtml
http://www.novotel.com/novotel/fiche...he_hotel.shtml