This doesn't necessarily have to do with Wessi vs Ossi - how many offers have you obtained? I have offers for the same services (2 bathrooms with level shower, 5.5 rooms, underfloor heating, solid house) according to the building description (ok maybe once 25€ tiles, sometimes 30, sometimes 23) for a 150m² city villa ranging between 160k and 222k without extras (roller shutters, heat recovery, clinker or monolithic etc). Even with the increase in m² I have values from 650m² to 3000m². With a ceiling height increase of 10-12cm also between 1300 and 2500€.
The price range is enormous even among only German providers (no franchise or so).
Then factor in that currently for medium loans ~250k it makes no sense to build according to KfW standards because banks offer a lower interest rate than KfW and you only have to build according to the Energy Saving Ordinance.
That easily adds up to 100k