House with 140 sqm living area, without basement, solid construction, gable roof - cost?

The construction company has been "on the market" for decades and is mainly active here in the region (Hochsauerland to Dortmund). I have also inquired with several former clients who have built with them (from shell construction to turnkey), the experiences and reports were mostly positive. We also gained this impression through several conversations with the construction company. Therefore, the question here is whether the quoted price is reasonable.

As someone already said, that sounds rather cheap, but whether it is still affordable is hard to say. For example, for the foundation slab, we were always offered something from 20cm upwards, up to 30cm. I also consider 15 cm to be very little. The same could apply to the carpentry work; are 24cm thick purlins used here, or do they only have 16cm? In general, the offer is certainly not overpriced, but for example, we paid about €100k for shell construction with roof (also approx. 135m², but plus basement!) and our foundation slab is over 20cm... Therefore, this is no bargain, especially since, it seems to me, savings are also made here and there.