Hidden cost: 45 hours of lost productivity while your workers figure out the differences with the new version of Office. Or $200 for two days of training.
More hidden cost: The next version of Office won't be free either.
Version differences have never been a problem. What is a problem, is when you have a few users, engrossed in the glory of open source, sending attachments that can only be opened by themselves...
But you are right about one thing, Office 2007 is wildly different, but for those who just want to type a letter, or create a spreadsheet, it only takes them about 45 minutes, not 45 hrs to figure out the major differences. Mildly aggravating, I had to create a vb script to force 2007 to save in 2003 format by default.
btw... just purchased 500 copies of 2007 for $28 each as an upgrade. $28 is well worth it, to remain compatible with the other 95% of the world. The 2% that are too cheap to purchase value for their money, cause about 75% of my trouble tickets.