look at the TCO page for the announcement [1]. The biggest cost savings Amazon is touting lies in reduced IT staff and total elimination of hardware costs. By their calculations, the licensing is only 3% or $3 per seat for a 1,000 person business unit.
That's the elimination of server hardware costs, though I can't imagine just chucking stuff in the cloud with no local backup. (Amazon has lost data before.)
There's still a client PC cost...
There are also the times when either Amazon (or Google Apps & Docs) or the network goes down and you have the odd thousand people sitting around wondering what to do. That's always lots of fun.
[1] http://aws.typepad.com/aws/2013/11/tco-comparison-amazon-wor...