How much does a mostly self-built house cost?

I simply assume now that you have to work at least 1 hour for a 50€ saving.

I know, you have already highlighted it in bold
However: if you assume a journeyman's net hourly wage of 50 €, unskilled workers on average have to work twice as much.
This thus drastically halves the saving... at least

That is not what the craftsman gets paid per hour, but what he can cost. And (hopefully!) cheap labor is not working everywhere..

I find 50 euros net hourly wage also relatively high. I have not seen such an amount in any offer so far.

50 euros is normal for skilled workers - here in NRW.

50 euros is normal for skilled workers - here in NRW.

Also for us in the "cheap" Lower Saxony

Here is a brief overview that I quickly found on Google:

13 EUR gross
11.74 EUR ancillary wage costs
17.06 EUR operational overhead costs
2.20 EUR entrepreneur’s profit (then taxed)
= 44 EUR billing rate
+ VAT
= 52.36 EUR

The craftsman is left with 8.61 EUR net. So that is just about 1/6.

It continues with savings, for example, if generous margins are added to purchasing items (or if the company simply has poor purchasing conditions?). Just as an example, my construction company wants just over 1,500 EUR for an enthalpy exchanger, on the internet it is available for just under 1,000 EUR (yes, the same one).