Cost per construction cubic meter: rule of thumb?

When it goes to extremes, any simplified standard calculation becomes increasingly inaccurate.

That is actually the crux of the matter. It's about the standardized calculation. It's absolutely not about any extras. In the first calculation by the builder or architect, it's about having a price based on the "standard." That's why you also discard things later.

All those subsequent comparisons like "my QM was this much and theirs was that much" are pointless.

In reality, it looks like this: you always take the calculation that results in a lower amount and then wonder why the money is not enough.

But seriously, as RobsonMKK already says, the crux lies in the extremes. With a standard ceiling height, it should actually come out the same, whether you calculate by volume or area. In the bathroom, there is only a bathtub, a shower, and 2 sinks. Whether the bathroom is 75m² in size or has a ceiling height of 7.5m, the calculation never adds up and neither scales with area nor ceiling height. With area, I need more flooring, with greater ceiling height more "wall". Maybe it makes sense to calculate your costs using both calculation formulas and then consider that the truth lies somewhere in between. I would go so far as to claim that these are not actually calculations but cost estimates with certainly 15-20% leeway up and down. This cost estimation is one with which the architect can say during the initial consultation which direction it is going. The actual cost calculation takes place much later.

In addition, large houses are generally cheaper to build per m², as you only need, for example, one heating system. Our general contractor requested one day after each meeting to calculate what the house would actually cost with this and that construction specification. For instance, we made the house about 10% smaller once, but with otherwise the same equipment, the price was still only about 4% lower. This already shows that you can simply estimate with values of around €1600 / m² or €350 / m³ as a first reference point.