I checked my records and my last Y ticket LAX-SYD-LAX cost USD 686 ai -- but it was a few years ago (I don't fly Y much). Currently you can get about USD 1000 with date flexibility, and USD 1300 JFK-SYD-JFK. Fares do vary by hundreds of dollars every few weeks (unlike most Asian routes). SIN-SYD/MEL has LCC options, so going that way can be cheaper but far less convenient (and limits baggage).