Tech Yes City did a video about windows 10 keys last week. While he didn't come up with an iron clad answer to the question about whether they are 100% legal it seems fairly safe to use. He also has a discount code to one of his sponsors for 12 dollar keys.
We CALLED Microsoft to Check if $12 Windows 10 PRO Keys are LEGIT... - YouTube
We CALLED Microsoft to Check if $12 Windows 10 PRO Keys are LEGIT... - YouTube