Building a house on a slope - From which point is it better to avoid using fingers?

I can tell you from my own experience that not only the slope affects your costs. Do you already know how your neighbors are building? We had everything calculated, obtained quotes to stabilize the slope (L-shaped stones, fill, etc.), and asked every neighbor except one how they are building. Of all people, the one who has the same challenge regarding the slope is now building differently. Which completely throws the planning overboard.

Your 40k€ should be enough though. This would have easily sufficed for our 21m x 29m site with a height difference of 3m to the street.

There is no significant slope across. Valley view towards forest landscape.