Hmm.
Prices in SF on AirBnb range from $50 (very low end) to about $80-100. Assuming you stay 30 nights at $60, that's $1,800. The apartment room he used to stay in probably cost him $800, so it's about $1000 more expensive to use Airbnb.
The publicity they have received just today has probably more than made up the difference in cost.