How much additional cost according to the structural engineer's calculation?

I see both parties as the problem. On the side of the builders, an explosive mix of ignorance and resistance to advice. And on the side of the construction company, dubious business practices where houses at bargain basement prices are sold to naive customers, only for half of that to be added on later as additional costs. I am building for the second time now, and neither of the two construction companies tried or pulled something like that on us.

Well, it was our fault for not informing ourselves better regarding the basement. It was stated in the offer (cover sheet) that depending on the type of soil, considerable additional costs would arise. So it would have been our responsibility not to have the soil report done only within the scope of the work planning but independently before the contract.

This is just an offer If I put together the last 20 pages correctly, then you are defending a company here that wants to sell you a basement without waterproofing and at the same time wants to charge you 30k€ for the waterproofing (black tank). Now it will at least be a white tank. By the way, this is not a protected, strictly defined term either. Here the quality/the mix of the concrete is relevant. However, the construction company is not the problem here. You simply have no clue about anything right now, so the general contractor does whatever he wants with you & that’s exactly what he’s doing.

and with which two construction companies did you build?

Completely regardless of your soil conditions, EVERY basement needs waterproofing. They tried to sell you a black tank for an additional charge, even though the offer says "including basement." Without knowing the exact contract text, that sounds highly unprofessional.