Most of the affected housing used to be government owned, sold at firesale prices after the CDU Diepgen government thoroughly wrecked the state finances with a banking scandal at the end of the 90s.
The large investor landlords purchased the homes and often didn't even do the bare minimum in upkeep. The real estate nevertheless appreciated massively because of Berlin getting more and more popular.
It therefore makes sense to nationalize this housing stock, and I'd see a value comprised of "original purchase price, upkeep and inflation-plus-3%-tied appreciation" as fair value. Maybe even less because they extracted 20 years worth of rent payments from the properties already.