Yes, Microsoft finally announced on there blog post the Bug Bounty rewards for the reporting the responsible disclosure to Microsoft.
There is much huge amount as like $100k USD, that are going to rewarded by the Microsoft to the researchers.
Security Researcher who will able to exploit and by the security on the new version of Windows 8.1 preview, then Microsoft will reward $100k USD to the researcher.
For giving "Defensive ideas that accompany a qualifying Mitigation Bypass submission" will be rewarded as $50 USD.
Additionally, After these bounties, there is one more bounty on exploiting the new version of Internet Explorer. Security Researcher who will exploit the Internet Explorer 11, integrated on the Windows 8.1 preview, he/she will be rewarded by $11k USD.
To take part in the Microsoft Bugs Bounty, you have registered on Black Hat Conference.