Price difference between ground slab and basement. Can it be estimated?

Can you roughly estimate how much the price difference is between a slab foundation and a usable basement?

One must not forget two things: first, the flatness of the property or the effort required to level the terrain, and second, the above-ground basement replacement room (which also consumes floor space ratio). In addition, there are groundwater conditions and the like; a basement is not automatically inexpensive just because you do not intend to make it livable. Building a basement on flat land only for the house connection room and otherwise a vacant dance floor never pays off. However, undertaking retaining wall contortions just to "save" a basement would be a Pyrrhic victory in effort and cost.

Omit the basement and make the house bigger. Building a basement can be like reaching into a lucky dip.

Skip the basement and make the house bigger. Building a basement can be like reaching into a grab bag.

I wouldn't generally agree with that. On absolutely flat terrain, that is certainly a good approach, but a basement can definitely be used meaningfully and would always be worth an extra cost to me. Especially bicycles, drinks, garden supplies, etc. are simply perfectly stored there. In addition, a laundry room and drying room are a nice-to-have, even if I might be a bit old-fashioned in that regard. Everyone has to answer that for themselves. I'm also not a fan of bungalows, even though I can understand that many dream of them. No stairs does not necessarily mean short distances for me. There are good and less good layouts for every house type.

On the topic of grab bags, I completely disagree. Today, a soil survey is mandatory, and you should inform yourself about your building area. Even if you really are the very first house in the new development area, there is plenty of information you can get by talking to residents, etc. Unless a bomb or an ancient grave is found there, you can keep the risk very minimal. Of course, if you always choose the cheapest soil analysis from dubious sources, you can't be surprised afterwards.

The decision about the basement should be made consciously, because it does cost quite a bit.

The decision regarding the basement should, however, be made consciously, as it does cost quite a bit.

Above all, omitting a basement from a certain degree of slope costs just as much as building a basement. And it must have a use, otherwise the price would be divided by zero. A house must fit on a plot of land in more ways than just by its footprint.

A basement is only worthwhile if you need the space, i.e. leaving it out would mean enlarging the house itself by that area. If you do not need the space and compare a house with X sqm to a house with X sqm plus a basement, it is never worthwhile.

A basement is only worthwhile if [...]

I fully agree with your calculation insofar as it is only presented two-dimensionally. However, a third dimension may also have to be taken into account: if I have to practically jack up my house by a basement height at even one corner, the "price advantage" of the "omitted" basement increasingly diminishes.