If and its still a big 'if', but if the RAAF did want 1 or 2 more C-17's, a decision would appear to have to be made pretty quick.
If I remember correctly they cost a bit over $200m each, two airframes plus spares and support would probably cost near enough to half a Billion, and the question would be, were does the money come from and would any existing capability have to be sacrificed to do that?
An argument could be made as to their value as humanitarian assets, which we've seen demonstrated on many occasions, maybe an argument could be made too for supplementary funding to assist in a possible purchase. I've also read that space at Amberley is also maxed out, if so, then more dollars would have to be put into expanding facilities there or the extra airframes could be based at Richmond, still I can't see it happening.
And of course too with the withdrawal from Afghanistan, you would assume the work load on the C-17 fleet would be reduced to a degree, maybe that is another thing, apart from the cost, that would go against an additional purchase.
Not that I'm against more C-17's, but if there was extra money floating around, which there doesn't appear to be, I'd rather see Air Lift Group get 3-5 more KC-30A's to increase the aerial refuelling capability, but that's another story!!