Arguably purpose built rental buildings have tighter control over the cost of that rental. Condo buildings on the other hand have inflated cost at the builder end because they have to be built fancier to entice buyers. That's why you only see "luxury" condos with names built. Then you have buyers outbidding each other, inflating the purchase price for those condos. As a result a much bigger amount of money is locked into that building doing nothing other than extracting interest and higher maintenance fees from the owner.

The reason I'm talking about buildings is because you can't solve the housing problem with houses. They don't work economically or environmentally. In that case the only relevant modes of housing are rental buildings and condos.