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.