[lxgr]

There is actually a path to $2 keys:

Most modern smartphones support contactless smartcards (a.k.a. "NFC"), which can be used as FIDO credentials. It should be possible to produce these for around $2 at scale.

They wouldn't work at computers, unfortunately (not even with an adapter, since desktop browsers and OSes don't expect to speak FIDO-over-ISO-7816-over-CTAP-over-USB), but with QR-based cross-platform flows now part of the specs, phones could pretty straightforwardly serve as readers for other devices.

If large issuers of ISO-based smart cards (e.g. banks or government authorities for biometric ID cards and passports) could be convinced to just throw a FIDO implementation on there (there's open-source ones available!), people could even use the cards they already own.