Linux might make headway for that cost reason now that we're in economic downturn, but right now Mac has over three times the users.
Linux began making headway long before the economy went South. The value proposition of low, or no acquisition cost is just icing on already very appealing cake.
As far number of users, I have heard OS/X has anywhere from six to ten percent of the *OS market* depending on which survey you happen to believe. I would say its not hard to believe that market use of Linux/BSD on the desktop rival this at least.
Honestly I am not sure how people are counting number of Linux/BSD desktop/laptop users since there is no license to buy. You download, then go using one copy of a download to install numerous times/instances. For all either us know there could be many, many millions of users.
Mac OSX is a very small part of the cost of a Mac. And all the major desktop GNU/Linux apps run on Mac OSX
According to the Apple site, a copy of Leopard costs $129. Not exactly trivial. though I am not sure what percentage of the cost of a new Mac is the OS.
I have no problem with Macs, or OS/X but for my money I can get a lot more value through commodity hardware coupled with Linux, or FreeBSD.
Many folks are discovering the same.