I'm just going off of my understanding of how wholesale prices and royalty calculations are typically set.
But let's accept your assumptions. In a revised reseller model with lower cover prices, in order for Amazon to make any profit the wholesale price on a brand-new release would need to be under $10:
$8 cover/wholesale price
$10 Amazon's price
$1.60 royalty rate (20% of the wholesale price / Macmillan's net)
In this scenario, it's unclear if Macmillan can still earn a profit, as the cover / wholesale price would need to be cut by nearly
65% in order for Amazon to just break even. I'm pretty sure that would wind up having an effect on royalties.
Overall though, I still think no one really knows the full effects of these changes, and unfortunately experimentation is likely in order. That's my story and I'm sticking to it.