Obviously using the MS developed framework is cheaper than building your own cloud network, but who knows how much cheaper than contracting with Amazon or Rackspace. Big publishers, at least, should be able to negotiate decent deals that cover all their titles, or Sony could negotiate bulk rates for everyone.
I'll have to find the quote, but during the early months of reveal, one of the developers said that the MS Azure platform is cheaper than any other cloud provider, cheaper than amazon or Google or rackspace.