Construction costs are currently skyrocketing

Chart on this, status as of yesterday for hot-rolled steel (something like sheet metal, rods like those used in tree mats is a somewhat different matter in terms of processing)
[ATTACH alt="stahlpreis-entwicklung-warmband-2016-2022.png"]72634[/ATTACH]
The steel price for hot wide strip continues its normalization, and so the spot price ex-works Ruhr falls to 940 euros per ton. Although this is the lowest level since January 2022, there is still sufficient downward room. The once major steel consumers in the automotive industry have become marginal players.

"The auto (industry) is dead," says a German dealer with regard to the weak steel demand of Germany's flagship industry to Platts. "They neither pulled as much steel as intended in the spring, nor will they reach their normal order quantities in the third and fourth quarters."

So it is still about twice as expensive as in 2019 but at least around 1/4 cheaper than in March.

Unfortunately, I have exactly the same impression... the drilling alone costs almost €45,000 (gross)... we also have an offer from another company for a ground collector (including identical heat pump + complete underfloor heating, however the heat pump with 5.8 kW, which is perfectly sized)... all in all it is just under €39,000 (gross)

Drilling + pump + heating from the other provider at €105k (gross) - who could and would want to pay that?!?

With an air heat pump, it probably costs around 20k. That you never make back the additional costs compared to the ground source is mandatory.