Windows 10 does not charge an annual fee. Microsoft principal Gabriel Aul promises

Since the launch of the Windows 10 Preview, it has been widely speculated that Microsoft will try to switch to a subscription-like charge similar to Office 365, which will obviously shake people's willingness to upgrade to Windows 10. Previously, Microsoft had said that users of Windows 7 and Windows 8.1 would be able to upgrade to Windows 10 for free during the first year. However, in order to dispel people's doubts, Microsoft Insider project leader Gabriel Aul has already issued a promise via Twitter —— Windows 10 will not charge an annual fee!

In other words, from July 29, 2015 As of July 29, 2016, Win7/8.1 old users have the opportunity to upgrade to Windows 10 for free. Users who upgrade after July 29, 2016 will only be charged for upgrading to Windows 10.

Gabriel Aul did not disclose the exact cost, but it is likely to be the same as the OEM version (such as the $119 home version and the $199 professional version). Of course, given Microsoft's efforts in promoting it, this round of upgrades may offer a special discount.
Since the concept of "WaS" has been proposed, many people have suspected that the company will switch to its subscription system for its own operating system. But in fact, it just means that the system update will be more regular and frequent (rather than a larger service pack update).