Silverlight's implementation of WPF runs on OS X already so presumably a lot of the ground work, at least for an OS X port, exists. I'd buy a copy of VS for Mac in a heartbeat.
The other thing is that I want to use OS X for everything else -- I much prefer using it for development above Windows, so while you may be correct in terms of sheer dollar value comparison, in terms of opportunity and switching costs it's quite higher than that for me personally.