A Full Win 10 PRO Key is around 5,- on Ebay.
Please note that this type of key may be an enterprise key. Now and then Microsoft catches on and disables them. If you happen to have a newer machine (since Windows 8) that has the key stored in the BIOS, then Win10 seems to happily install on those (doing a fresh install).
This is from personal experience:
Since using Windows 10, I've not had to reinstall to "clean out" a system*, both personally and at my place of employment. Of course, I'm not a user that constantly installs a myriad of software. I have migrated several Win 10 systems from rust platters to SSD's without issues. In the end, this seemed faster to me since I did not have to re-install any other software on the system.
*Not quite true: On a initial system purchase, I may re-install Windows 10 in order to remove any unwanted vendor software. From then on it's smooth sailing.
Last edited: Feb 24, 2021
Disclaimer: Just because I give an answer (short or long, but especially long) does not mean I know what I'm talking about. Any corrections are welcome, since corrections lead to new knowledge.