Which is an insane amount of money these days.
Plus, MS puts you on a treadmill to keep the revenues running.
Linux? I've never bought a PC (think the hardware is lame and won't pay the MS tax) so I can't speak to its general usefulness - but the Linux distros for various macs (which I have installed) were so fiddly and lame as to leave me wondering what the noise was about.
My opion of the apps on linux - they kinda suck. The dev environment - ditto. Even nextstep isn't ready for prime time.
OS X is a nice supported commercial unix with good dev tools and a good suite of apps. Plus I like the hardware. Its not a fiddly thing that drives me nuts getting it running. It just works out of the box. So I like it. What a surprise, we're in the Mac forums and I'm saying I prefer OS X to any other OS out there. Try not to act too surprised, OK?
I do happen to have a linux box at the moment as a workgroup server. Its running resin to run the devtools.org wiki, its our cvs over ssh server, but we never use an interface more elaborate than bash on it. For what it does, its fine. But I don't think I'd use it for much more than that and I'm probably going to replace it with a solaris box.