How expensive will this house be?

You won't get houses for €1000/sqm. A little googling is enough to find appropriate values.

Unfortunately, the online construction calculators and rule-of-thumb formulas tell me exactly the opposite, so please forgive me for assuming such a price.

Then you filled them out incorrectly or just let the shell construction be done ;)

That sounds as if worldwide there were only a handful of plots where you could build a small square house *scratches head* and why should it be twice as expensive as a comparably sized "conventional" house? Because of the slanted walls? Please enlighten me

How many plots near you can you spontaneously find that are at least 18m x 18m?
And if it suddenly is only 17m wide, your whole planning is wasted.

Are you aware that many plots don’t have four corners or many are not rectangular?

What will you do if you find a great plot on which, unfortunately, your house does not fit? Reject it? Or replan? In the latter case, you might as well only plan after the plot is known.

Since all boundary conditions are still missing, I would base a what-if calculation on the worst case. And in my calculation, that would be closer to 2000€/m².