Per this thread in Flyertalk, Air Canada is offering returns between Hong Kong and San Francisco via Vancouver for as little as C$500 (HK$3,080). The same price also works to LA. There is good availability in January and February. The flights go through Vancouver. It seems that you cannot have a stopover (24 hours or more) there at this price, but you can have an overnight layover.
Bear in mind that Air Canada packs in the seats on its planes and its service is not at the Asian standard, so even at this price the fare may not be attractive for everyone.