I wonder.. With iOS7 being a free update and reaching over 70% of the iOS user base in a month, and Windows 8.1 being a free update to all existing users.. Will Apple still charge 20€ for Mavericks, or will they deploy it as a free OTA update for existing Mountain Lion users?
Looking at iOS giving if out for free would be a logical choice. Almost overnight, the majority of their user base will be running the latest OS, giving Apple both a nice number to share to the media, and developers a nice user base to develop for.
I mentioned Windows 8.1 not because it's a competitor (it's not) but because it does something unusual. It gives existing users new features for free. Something that wasn't really allowed for regular sales and is only allowed for subscription services according to US law. It's the reason Apple used to charge a couple of bucks for iOS updates for iPod touch users. But new that Microsoft apparently had found a way to give away a free update to their OS with major new features (skydrive integration, split app multitasking,..), there's no reason Apple couldn't do the same.
So my bet's on Apple giving away Mavericks for free next week for all Mountain Lion users. I could be wrong. I could be right.