My prediction is that Mavericks will be released as a free operating system upgrade, while the next version of OS X will be $20-30. Mavericks is a quick incremental update. I think Apple wants everybody to upgrade to Mavericks ASAP. The way they are developing it is pointing towards it being the next Snow Leopard (Under the hood improvements, long beta testing, light on new features, performance and battery optimizations). It's the new stable OS version. It's cleaning up the mess that was made with Lion. Not that Lion wasn't a good upgrade, but it wasn't the smoothest upgrade, and it introduced performance issues. The next version of OS X might be the next Lion in terms of UI changes and more drastic changes to the way the operating system works. I personally think Mountain Lion should have been free, too, but it ended up being $20, so who knows?