Calculated construction costs incomprehensibly high

That's true, but he can hardly charge a material price above the recommended retail price, that would really put him in a difficult position to explain.

On Monday I have the sample selection, then many things should be clearer...

I wouldn't be so sure about that. I do think that he adds surcharges on top of the recommended retail price. Good luck anyway!

I also installed the sanitary fixtures myself. Above all, the shower partition would have been quite expensive. The walk-in shower wall 140x200cm with 10mm safety glass cost me €240 + 2 glass suction cups for transport and installation. The extra cost for the sanitary items alone was in the four-figure range.

And for the freestanding bathtub... I could have gotten a car for that. And it is more or less simply placed there...

Did you connect all that yourself then?