Even with permission, I recently looked into getting a place built, and found a company offering to build them for "€100,000*", where the asterisk was: this does not include the land itself, land survey to find existing underground structures, earthworks needed before construction, utility connection costs, energy costs during construction, plastering, kitchen installation, bath or shower installation… add those in, and the €100k becomes around €500k with half the difference being the land.

The land costs in turn are, so far as I can see, mainly dominated by needing some kind of road there already. But I don't know if that's cost per se or a supply/demand thing setting price, with the difference being someone's profit.