According to
this, Apple has paid out over $1Bn to app store developers, which would equate to a take of ~$430M for Apple. By comparison, in just Q4 of last year, they made
$4.32Bn profit on $20.34Bn revenue. Of that $20Bn, ~$1Bn was iTunes store revenue.
This time last year, Apple were describing both the iTunes and app stores as
at bit over break-even.
Apple views selling content primarily as a way of increasing hardware sales, the opposite of Amazon, who view selling Kindles primarily as way of increasing eBooks sales. (Hence both Apple and Amazon benefit by having a Kindle app on the iPad.)