Microsoft has the monopoly. They own the market and can charge what they want. Apple needs to be cheaper to gain it's footing on the OS realm. Eventually a $130 OS will gain ground over the $400 one no matter how many people have the latter.
Apple is not competing with anybody on the OS market. If they were, they would make OS X also available for non-Apple hardware. But as it is today, you need to buy one of their premium priced machines to get your hands on OS X. And that is exactly the reason why their operating system won't gain much more market share than it already has - people want to freely choose their hardware, they want customizable and upgradable machines AND they want them cheap. Apple does not offer anything for the mass market except their iPod gadgets.
Furthermore, for a magnitude of reasons, Apple's platform is not ready for the enterprise market either - and that is where the real volume of soft- and hardware is sold.
However, have I mentioned that almost all Intel-based Macs also have a Windows installation in one way or the other? (BootCamp, VMWare, Parallels.) What in reality happened that Microsoft was able to increase their Windows sales because the last bastion against Windows finally opened its gates for it. This is the main reason why Apple has been able to sell more hardware units than in the years before their switch to Intel CPUs.
About the price: You can buy Systembuilder versions of Windows for less than the retail price of OS X. Since the OS X retail boxes come without any support, they have to be compared with Microsoft's systembuilder versions - the Microsoft retail boxes include two technical support requests (and technical support points don't come cheap).
Unless Apple opens Snow Leopard for compatible(!) third party computers (built after Apple's technical specifications), they won't ever grow out of their 10% niche. And don't tell me they're comfortable there. Thanks to iTunes and the iPod, Apple Inc. has finally become a mass market company, and it is their responsibility towards their shareholders to keep growing. And they could grow much faster if they dropped their last century computer business model.