Well then, you make 100cm for 10,000 EUR but have a full floor.
I think this was already asked but if there was an answer, I must have overlooked it: Are you bound to Bien-Zenker? Already signed? It doesn't have to be Bien-Zenker.. If not and prefab houses are not mandatory, why not go to a regional solid construction general contractor and get an offer for a "normal" two-story gable roof house. I could imagine that would be cheaper.