You can roughly calculate the following:
House with foundation slab turnkey 210,000
finished garage 10x3m - 10,000 euros
excavation work, cistern, pipes 18,000 euros
land xxx euros
miscellaneous another 20,000
then you are, you you you through. On the subject of value comparison solid vs. prefab I can only say that I know someone who works in a field that looks far beyond the horizon. There I was told that in the coming years procedures will be developed to completely renew prefab houses through simple renovation after 60-70 years. That means your children will buy, for example, in xx years for 30,000 euros a complete renovation of the exterior walls and it's done. Possibly in the future there will be materials that are thin and strongly insulating. Here you can rely on the German industry and craftsmanship without worries. With a prefab house you can’t go wrong. As I said, don’t buy from cheap providers but from premium providers. The consumer protection association surely gives tips.