I can't remember if I posted this before - I have booked a mixed class flight through VA flying SYD to LHR (Prem Econ on the long leg, Econ on the shorter leg) and was offered the auction at the time of booking for the VA flight only (SYD - AUH). The minimum bid for an upgrade to Biz is $600.
very cheap, but I suppose reflects the higher fare yoo paid for Premium Economy in the first place. Etihad, of course, don't offer PE. At least not on the Kangaroo routes.