From the Q&A session after the reveal I think I have a feeling for what Microsoft's pricing model might be going forward and it's basically this - If you are willing to be on the bleeding edge of their codebase and put up with any issues that may generate and give feedback where you can then the OS will be a nominal/no charge, if you require a more stable OS (business/enterprise/old people) then you can buy more traditional updates and only the critical and tested security & stability fixes will come to you. There may even be a subscription model hidden away somewhere in the future.
It's basically the enterprise linux distro model.