http://arstechnica.com/gadgets/2015/01/windows-10-free-for-all-windows-8-1-and-windows-7-users-for-first-year/
“The biggest news is that the new OS will be completely free for current Windows 8.1 and Windows 7 users for its first year of availability—after that time period has expired, OS upgrades will presumably need to be paid for as they are currently (though Microsoft was less than clear on this point, it made no mention of a paid, Office 365-style subscription for Windows upgrades). The Windows 10 upgrade for Windows Phone 8.1 users will also be free.”
