I know that it's probably old news by now but an article in Network World claims that the Total Cost of Owning (TCO) a Mac is about half of what it costs to own a PC. From Business 2.0:
World columnist Winn Schwartau runs the numbers on what it truly costs to run a PC and a Mac in a business, and finds that a PC is nearly twice as expensive as a Mac when one takes support costs into account. Antivirus protection, firewalls, and IT labor to keep a PC secure and operational add up to a bill of $1,300 to $4,000 a year, according to Schwartau. Of course, there's one hitch in Schwartau's calculations: As the Mac gets more popular, it's becoming more of a hacker target, and protecting against those new threats could drive security and support costs up over time.
For the full article with all the methodology and details check out Network World's "Going above and beyond"by Winn Schwartau. It's far from perfect but it's a good read.