Wie viel wird das in Luxemburg wert sein?

With the legal cap, probably not much if it was bought before 1960 and if no notable renovations were carried out.

https://gouvernement.lu/dam-assets/documents/actualites/2021/04-avril/23-brochure-simulateur-calcul/Brochure-Calcul-du-plafond-legal-du-loyer.pdf

I live in an old building (1930’s old) and am paying about 14€/sqm.

Before that I was renting 50sqm for 1200€.