Other payments processors charge 30c + 3%, so it would be interesting to see how it would work here for smaller payments
Between the two approaches, the costs break even when 30c = .27*cost.
Apple's 27% exceeds the 30c whenver the purchase costs less than $1.11. 27% of .99 is 26.73c.
So, this is a strong case Apple has calculated the other processor's charges for all app sales of .99 or less. It seems they are trying to charge more than normal processors for all micro-transactions.