Windows 10 will be a free upgrade to Windows 7/8/8.1 for a period of 1 year. After 1 year, you will have to pay for upgrade. How much is uncertain.
Microsoft calls windows 10 for a service, and by giving Windows 10 away for free for a period of 1 year, they hope that more than 20% will use Windows 10. Only 20% are using windows 8/8.1. Over 60% are using windows 7 and less than 20 procent are still using windows XP.
I have been using windows 10 tecnical preview for a month, and I don't see major changes from Windows 8.1. It's like 1 step forward and 2 step backward. Microsoft needs to make an OS like Windows 7 with enhancements and up to date technology. Windows 8 was a failure....like Vista and ME. Windows 7 is like 6 years old, and have 1 servicepack. That's just amazing. Windows 8 is 2 year old, and until today got 2 major servicepacks. It says a lot.