Mavericks is free.
Its system requirements are the same as for Mountain Lion
It is also a security update:
I surmise that Apple does not intend to provide security updates for older OS versions. At least not for 10.8. If you want to get patched, start upgrading to 10.9…
While it was fun to speculate about Apple forcing us to upgrade to 10.9. It was just speculation. I now believe I was wrong and I expect Apple to release updates for older versions similar to its past behavior.