As for IT costs - I have worked in several companies over the years with both UNIX and Windows server rooms. Being a UNIX person, I may be a bit biased, but my personal impression is that supporting Windows servers is a lot more painfil than supporting UNIX/Linux - at one point I supported some 50 UNIXes alone, while the roughly similar number of Windows systems had a team of 5; I had a pretty relaxed daily routine, but they were always overstretched. Not because they incompetent, I learned a lot of generally useful stuff from them, but so many things in Windows seem to require either clicking through graphical interfaces, system by system, or require a specialised, graphical tool, where I would just run a few scripts from a command line. The power of tools like ksh (or bash), ssh, sed, grep, find etc should not be underestimated.
The other thing I have heard increasingly - from Windows admins themselves - is that Windows is just such nightmare to handle. I wouldn't know - I left Windows behind as soon as Linux became viable, and that's a long time ago.