I will visit a friend in Berlin and I would like to know a good german restaurant not too expensive (20€ per person), in the center of the town
Max und Moritz is a great, authentic German restaurant not too far from the center of the city that has native Berliners' stamp of approval.
Restaurant und Bar Keyser soze is awesome! Great potatoes & beers you'll love it!! :) and this district (Mitte) rocks!
Zum Schusterjungen is pretty solid and affordable. It's in Prenzlauerberg which is one of my favorite neighborhoods in Berlin. Great non-touristy, leafy hood to walk around.