You can buy a Windows license key for $9. If the link does not open, then look at
Just to note that those are not legit retail keys but instead ones obtained in many "creative" ways and usually shoulnd't even be sold. I mean, they do work and might be ok for private user, but for example if you have you own business and are using that cheap key and inspection visits you, you will get fined.
But the question arises as to why people even pay for Windows if we know that at least version 10 was free to upgrade, and almost fully functional even without a license.
Why should I mess around with cheap keys or pirated Windows when I can get the retail key for ~100 euro? Imho that amount of money is not worth the hassle. It was when I was 14 years old and I had different priorities (like the amount of pirated games I can buy instead paying for Windows) but now I just want things working properly without any issues.