Construction costs are currently skyrocketing

It doesn’t always have to be the luxury version of a central controlled residential ventilation system with an enthalpy exchanger. I can’t quite follow your calculation right now. Over what period are you calculating (per day?) and is the 10 degrees the temperature difference? I can guarantee you that even in mild Konstanz over the winter, the indoor-outdoor temperature difference is well over 15 degrees. Where we are moving to, we have average temperatures below zero from December to February, so more than 20 degrees difference. And then we get completely different values.

I have a central controlled residential ventilation system with an enthalpy exchanger. Actually, for most people, 3-4 decentralized fans with heat exchangers would be more than enough. Bedroom, bathroom, and utility room (because of condensate). It might cost about 1k instead of 8-11k. My calculation of heat losses was just a quick, rough example. For example, I still like to sleep with the window tilted in winter or briefly air out in the morning. In winter, you also have to be careful with ventilation systems so that the humidity doesn’t drop too much; I know people who only have 30% humidity. But with a controlled residential ventilation system, you are always safe that the masonry is perfectly dry. Independently of the ventilation system, for a new building I would always focus on summer thermal protection rather than the last 2-3% of heating costs.

Actually, for most people, 3-4 decentralized fans with heat exchangers would be more than sufficient. Bedroom, bathroom, and utility room (because of condensation). Might then cost about 1k instead of 8-11k

Which provider offers a decentralized ventilation unit with heat recovery including installation and connection for €250?

I have a central controlled residential ventilation system with enthalpy exchanger. Actually, for most people, 3-4 decentralized fans with heat exchangers would be more than enough. Bedroom, bathroom, and utility room (because of condensate). That would then maybe cost 1k instead of 8-11k.

My calculation of the heat losses was just a fixed, roughly estimated example.

For example, I still like to sleep in winter with the window tilted open or ventilate briefly in the morning.

In winter, you also have to be careful with the ventilation system so that the humidity doesn’t drop too much; I know people who only have 30% humidity left. For that, with controlled residential ventilation, you’re always safe that the masonry is perfectly dry.

Regardless of the ventilation system, I would always focus on summer heat protection in new buildings and not on the last 2-3% of heating costs.

Even the cheapest decentralized ventilation units (at least not those with heat recovery) are not available at the above-mentioned price. Not by a long shot.

Personally, I have been monitoring my temperature and humidity since August, since then between 50 and 60%, the low point was just over 40. (central controlled residential ventilation with heat recovery without enthalpy exchanger)

From my experience living in a new building apartment (solid masonry construction), I would neither do without controlled residential ventilation in my own apartment nor in a rented apartment. Almost no one ventilates correctly, living comfort is lower, energy consumption is higher. The risk of component damage (mold) would be too high for me. These were and would be reasons for me as a tenant not to want to stay living there forever. The aforementioned apartment was "musty" for over 2 years. Proper ventilation was not possible for me (out at 7 a.m., back at 7 p.m.)

You are definitely right about summer heat protection. It is still too often neglected and people wonder why the place overheats in summer.

Which provider offers a decentralized ventilation unit with heat recovery including installation and connection for 250€?

That's correct, it already costs over 400, without installation. Still significantly cheaper than a central system. And can be retrofitted later or used for existing buildings.

So even the cheapest decentralized ventilation units (at least not those with heat recovery) are not available at the aforementioned price. Far from it.

Personally, I have been monitoring my temperature and humidity since August, since then between 50 and 60%, the low point was just above 40.
(central controlled residential ventilation with heat recovery without enthalpy exchanger)

If the house is less than two years old, you probably still have enough residual moisture in the building. In our office building, after two years, the humidity in winter dropped to 20-30%, which means we have to set up humidifiers in winter. Since then, the wooden staircase has developed elegant cracks.

But only half as good. (rough, personal estimate ;-) )