There is a lot of price competition currently on flights between HK and Los Angeles. The latest cheap offer is from American and United. Per this Flyertalk thread, return flights in economy class are available from around HK$2,900 (US$370) all in. You may need to dig around for dates on which these prices are available.
This was a cash ticket. We have highlighted very cheap American fares on this route before, and with the current fare competition on the route between incumbent Cathay (whose economy class on this route we reviewed here) and relative newcomers American and Hong Kong Airlines,