§556d, 1 of the German civil code.
https://www.gesetze-im-internet.de/bgb/__556d.html
It's illegal here to demand a rent more than 10% higher than the "ortsübliche Vergleichsmiete", which is approximately the local rent averaged over the last 6 years (exactly defined in §558)
More generally, you cannot simply ask for astronomical prices for anything in Germany just because you know that people are in a position where they have to pay them, that's "Wucher" (racketeering) and against public decency, which is generally illegal according to §138.