For everyone not familiar with German rental markets: Berlin is quite cheap compared to other big and small cities. From cities as big as Munich to smaller ones like Mainz, most cities are more expensive (except for those in the former GDR). Cheap(er) rents are part of the reason why people move to Berlin - the more the Senate does to control rent, the more people will want to move to Berlin. The only way is to increase supply - or rebuild the wall ;-)