Consumer purchasable system builder/oem keys are at a minimum $80, usually $90. These are limited to one PC only.
Retail licenses are more of course, but can be freely moved around between PCs as you upgrade.
This is not correct. As of Windows 10 a license is "wedded" to a motherboard upon installation and is good for the life of that computer. You cannot transfer a license to a new machine.
If that "machine" needed a new motherboard, and everything else remained, you would need a new license as Microsoft has determined the definition of a computer is a computer motherboard. You can change anything else, but if a new motherboard ID is detected, it would deauthorize.
In fact, I had a machine that I installed Windows 10 on and I needed to clean install the OS. As soon as it was on the Internet it authorized because it matched the board ID with the Windows database.
Part of this is because Windows 10 is the last version of Windows ever. It will evolve, but will always be called Windows 10. The days of moving a "key" from machine to machine are gone.