The difference between current demand, and the increased demand that would exist at a lower price.

I just mean, there are lots of people that would move to SF if they could afford it. This mass of potential immigrants will counter an attempt to just build your way out.

Unless you mean a real substantive change to the city e.g. building 30,000 apartments/year.

But that amount of money is astronomical. People are buying the apartments and leaving them vacant as an investment, rather than using them as intended, as a residence for a person living in the city.