Buy windows 8 now or wait for Windows 10

I wonder if I should upgrade to Windows 8 or not. I am thinking that if Windows 10 will be release in the future, I can upgrade to it for free so I want to know if I will purchase Windows 8 or just wait for the release of Windows 10. And by the way, what do you think about the cost of Windows 10 and if I upgrade to Windows 8, is it just for one time or do I need to install Windows 10 again in the future?
