Microsoft announced the first pricing information for Windows 10 at its preview event yesterday. 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).
"Once a device is upgraded to Windows 10, we'll be keeping it current for the supported lifetime of the device," said Terry Myerson, executive vice president of the Operating Systems Group. "With Windows 10, we think of Windows as a service... The question 'what version are you running' will cease to make sense."