secondly if the A380 is cheaper than the 777 per seat mile im sure it would only be when full. Flying around a 280 tonne empty weight aircraft with empty seats would send the seat mile costs thru the roof in my opinion.
You could probably fly a Navajo across the pacific for less cost than either empty.
Aircraft selection is about finding the right aircraft for the route, you need to get the range and payload compromise suited to the market. The aircraft is purchased based on attaining an average of at least 80% load factors on reasonable yields. If you can not achieve that the aircraft you bought is too big and you should have got the smaller one (A330/767).
Frieght is a small part of QFs income (read latest anual result), their focus is high yield passengers. VOz try to subsidise their operation with freight so the 777 may be better for their ops, although any ops in and out of the US would be better served with dedicated frieghters due to the volume. The dedicated frieghters can offer a much lower cost to frieght customers as they don't have all the frills a passenger operation requires.
The 77W has a 15-20% higher fuel cost per seat over the A380 according to Tim Clark (President of Emirates).
Why did they order another 30 aircraft (777-300)?
They also ordered 32 additional A380, which means both aircraft are proving successful.