For background, I work in the construction industry. You're right that most of the cost increase isn't because of the bill of materials, but that has never been the biggest cost in homebuilding (or at least not for the last 100 years). It used to be perfectly doable to build a multi-unit for $150k per door, and now it's difficult to keep it under $300k (note that this is in a midsize city, those costs are obviously much higher in a larger one due to far higher cost of land). By far the bulk of that increase is labour and subcontractor costs. I leave it to you to do the math on the rent required to turn a profit on a $300k apartment.