That was a few years ago, based on Exchange 2007 and 2010 -- meaning using fibre channel SAN for storage, tape backups, F5, etc. If you implemented today with Exchange 2013, you'd probably be able to cut costs 30% or more. There are a ton of costs areas embedded in there.

If you're operating a simple (ie. no calendar, no hooks into other services) service with custom code, open source infrastructure, and no (or minimal) external licensing, you can absolutely offer a $5/year 100MB mailbox profitably. They are making higher margins on the enhanced plans as well.