The Microsoft OS team is hoping to get as many Windows 7 users moved to Windows 7 Service Pack 1 and Windows 8 users to Windows 8.1 Update in preparation for (hopefully) getting them to move to Threshold once it is out. It's still early in the Windows development cycle for Microsoft to have decided on packaging, pricing and distribution, but my sources say, at this point, that Windows Threshold is looking like it could be free to all Windows 8.1 Update, and maybe even Windows 7 Service Pack 1, users.
Microsoft is basically "done" with Windows 8.x. Regardless of how usable or functional it is or isn't, it has become Microsoft's Vista 2.0 something from which Microsoft needs to distance itself, perception-wise. At this point, Microsoft is going full-steam-ahead toward Threshold and will do its best to differentiate that OS release from Windows 8.