The QF codeshares on AA are exorbinantly priced; compare LAX-SYD on AA vs. QF and AA is consistently 30% higher. So a good tip for those wanting an AA codeshare is to originate from some non-QF gateway city. Of course this is still more expensive than buying 2 separate tickets (in your example, DEN-LAX-DEN is often under $200 thanks to Frontier, while LAX-SYD-LAX can be as little as $700, though more often $1000).