These keys will never be $5 but pure U2F only keys are almost already there. You can get U2F keys for 8-10$ already.

Sometimes they do. They did when they launched yubikeys on github. You could get a yubikey with github logo on it for $5, limit 2. They were U2F only, but I still purchased two for the novelty.

Presumably they sell at their current price, why drop it?