OS X is free now on the condition of using only Mac hardware. Windows is the only OS that isn't free and their market share has been gobbled up by Apple in the last decade or so.

I think the only place where Windows is still pricey is the enterprise, but Apple has made it a point to give away their OS completely free to everyone. Apple got out of the server game mostly because Linux has and will take over in the long run (especially with platforms like docker which should make it easier to deploy apps).