Windows 10 is not only going to be free, but it appears to be going to the same model as Sonar - a perpetually and continually updated platform, rather than a series of versions:
http://blogs.windows.com/bloggingwindows/2015/01/21/the-next-generation-of-windows-windows-10/ "This is more than a one-time upgrade: once a Windows device is upgraded to Windows 10, we will continue to keep it current for the supported lifetime of the device – at no cost. With Windows 10, the experience will evolve and get even better over time. We’ll deliver new features when they’re ready, not waiting for the next major release. We think of Windows as a Service – in fact, one could reasonably think of Windows in the next couple of years as one of the largest Internet services on the planet.
And just like any Internet service, the idea of asking “What version are you on?” will cease to make sense – which is great news for our Windows developers."