Considering the quality and the fact that Win10 actually serves you adds and sells telemetry information, I think the price should be even lower.
The consumer has the responsibility to check the validity of the product, sure, but this is ridiculously unfeasible for digital licenses. A lot of companies actually do buy these 10$ keys and I don't blame them.
They haven't provided one, but I'm guessing if someone provides a source, it'll be a link to Microsoft's ads products. They'll argue this information _could_ be used to target users for advertisements, even though it might not be.
The consumer has the responsibility to check the validity of the product, sure, but this is ridiculously unfeasible for digital licenses. A lot of companies actually do buy these 10$ keys and I don't blame them.