I'm guessing that Barnes' salary should be close to enough to pay Izzy if he wants to stay (assuming that he receives some kind of ARU top-up). I would think that Drew would be taking a pay cut, too, unless he plays the house down and performs well internationally. There may be more money floating around than anticipated, particularly if Cheika refuses to overpay players.
Appropriately valuing players is essential in any salary cap situation, but doubly so in contact sports where injuries necessitate having significant squad depth and balance. The best NFL teams, for instance, will let good players walk rather than overpay them to stay. Often, there are guys 80-90% as good that will play for 25% of the money. From the dribs and drabs the media reports, Cheika seems to grasp this pretty well -- although the trick, of course, is in how well you do that valuing in the first place. I'm not an expert on this subject at all, but evidence from Leinster suggests that he is pretty good at it.