It's crazy easy to find warezed copies of Windows 10 on U.S. Amazon ($12 for somebody to burn a DVD of their bittorrent download, $15 to add a key).

By contrast, searches on AliExpress for Windows OS in any form dead-end hard. And it's not for lack of consumer interest either; Enter "Windows" and autocomplete suggests all possible combinations of "Windows", "10", "7", "OEM", "Key", etc.

I hadn't spent too long at this because it was out of idle curiosity rather than desire. YMMV.

Within the U.S., legit (albeit greymarket) OEM Windows start at $80 on Amazon.

Taobao shows less restraint w/r/t Windows 10 warezing. US$70 buys you an OEM key, but for US$1.50 you can have somebody unlock your Win 10 installation by remote administration.

Google Translate leaves service descriptions ambiguous, so don't put a lot of trust in the above.

Ebay wins for market madness, internat'l sales of Win 10 keys and downloads are a riot of prices. US$3 and up. Mostly via UK and Canada.

https://www.youtube.com/watch?v=up863eQKGUI