When is it worthwhile to demolish an old building for a new construction?

The only reason not to tear something like this down would be if it is no longer allowed to build like this. Because of boundary distances, outer areas, etc. Even during a complete renovation, you cannot live inside, at least not with 80m².

How much does a demolition approximately cost?

Depends

Amount of rubble
Pollutants
Disposal costs landfill
Transport

Just post a site plan of the property with the house. Then you can say more.

House 50 A thin walls, flat roof, ground floor and first floor each 40 sqm, detached, no basement, only strip foundation, is to be demolished and transported and disposed of over a length of more than 40 m, 3.5 m wide, paved driveway.

A demolition would certainly make sense. But. It is exactly on the border. Demolition would then mean the loss of this development. It would have to be clarified whether the old house could perhaps be preserved and integrated into a new building to maintain the status.