Actually the 2 biggest costs in running an airline these days are fuel and wages.
There's no doubt about it that a newer more fuel efficient aircraft would help the bottom line but if you looked at the cost of wages for QF mainline pilots compared to the costs of wages for the Jetstar pilot group there would be a huge difference.
It's not only the money the airline pays for wages it's also the super,holidays,LSL etc..and the difference would be significant.It's not only that but the efficiency of the two awards.So it's not just looking at a 2,3 or 4 person crew it's the costs of both groups you would have to look at.
If this was not an issue why did the company go to the trouble they have setting up the Jetstar model with different pilot groups?