San Francisco needs tens of thousands of new homes to help relieve soaring housing costs. Ideally, a large swath of these homes would be affordable for lower-income renters. But idealism has an eye-popping price tag, according to a new City Hall report. To meet affordable housing goals in its 2023 blueprint, San Francisco would have to spend nearly $3 billion a year for six years, and that scenario also requires other sources, such as private, state, and federal funds, pitching in the same amount.