Actually, I think that number is really light.
The simplified math that I used for business cases:
Power users - PC = 3 years, Mac = 5-6 years (coders, graphics designers, engineers)
Light users - PC = 5 years, Mac = 7-8 years (these being the ones only doing web surfing, office apps)
And, yes, those are the averages - IRMV.
btw - business casing a Mac for the "techies" is easy - typically the first thing they do with a new system is blow away the corporate windows install and put their favourite flavour of Linux on it. Then they start tweaking it. They all love OS X/MacOS as it's BSD based. Just give them more RAM and use VMWare images of all of the Linux distros they like (and have revision control - so they all get the tweaks/improvements that are made).
The Mac pays for itself in the first year (the alternative, you lose over a week of productivity from that techie as they "tweak" their image).
And, yep, I've also managed service desk employees. Calls from those using Macs were very rare indeed.
Back when I worked at IBM in the early 90's, we figured the cost of a support call (8 min average) was $35. (that's the wage, infrastructure, training, etc, etc). It took very few support calls as time went on until you lost your margin on a PC.
That was then....I'm guessing the costs are considerably higher now.
Oops....and I didn't even touch on software updates. That's another huge cost. In a corporation, they're all managed (and there are thousands of patches for windows each year....not so much with the Mac). Each patch has to be reviewed and approved before you allow it to go out. And, then, you really should be staging each "bundle" of patches - to select groups, to minimize potential for impacts. (which DID often happen - far too many apps out there to break).
Ok...I'll stop there.
