Well, if the house should be worth anything, it won't be that cheap, yes. What you can find tons of is stuff like "Denkmalgeschützter Bauernhof Brandenburg" for really cheap, because the renovations will cost you big time. Same without Denkmalschutz (historical building protection), just not that cheap. A newly built house will cost far more than 100€/m² because building houses is expensive after all, and as you said, usually the ground is worth almost as much.