I thought these numbers must be off but the YubiKey HSM really is $650 [0]. What justified that price? Can someone walk me through the typical use case/application for these?
If you truly care about the encryption on your servers, for example, then normally your server can't boot/decrypt the drive without an HSM (which you aren't supposed to leave plugged in). You give one to your CEO, your CTO, and your CFO, for example, and you require that at least two of them are present to boot your servers (to prevent attack by theft).
There are more fault-tolerant possibilities out there (e.g. Hashicorp Vault), but they're generally bootstrapped from HSMs.