I used OS X Mavericks as my main OS from beta 2 to beta 8. I'm back on Mountain Lion until it officially goes in the Mac App Store, and I have to say, there's a lot about it I miss on a fairly regular basis. I'm not going to go into great detail about it because of the NDA, but I will say that I'm excited to get my productivity, or at least my sheer operational speed, back up to where Mavericks put it. It really is a testament to its quality that a beta release from Apple, quirks and all, integrated itself so deeply into my workflow and improved areas I actually used.
My guess is that it's going to cost $19. However, we haven't heard the last of Apple for 2013 yet, and there's an event where they'll be introducing the new verison of iWork. Maybe that will go free for new Macs as well?
Maybe the whole OS is free? I highly doubt it, but it's always a possibility: Apple doesnt make their money on software, and generally prices it politically rather than for direct gain.