Klartext have some very reasonable prices and come highly recommended by people, particularly for the beginner and mittelstufe levels.
Prices: All courses last 5 weeks Morning Classes (20 hrs/wk, 5 d/wk) €330 Afternoon Class (16 hrs/wk, 4 d/wk) €260 Evening Class (5 hrs/wk, 2 d/wk) €100
http://www.klartext-zentrum.de/
See also: