You're talking about purchasing cost, there is additional cost as well. Who supports it,how much more does it cost to support. How easily can you issue new yubikeys,what is the cost of that delay? Do you still keep passwords or hope people keep their yubi's in a secure place?

Business cost example: some important guy is making a business deal but he lost/forgot his credential and can't login to show a presentation. If that credential is a password or TOTP key, he calls helpdesk and gets it sorted out. But if it is a FIDO key and they are on the other side of the planet (or a city close by where you have no support staff) that can humiliate not just the person but your whole company. Are hackers a bigger risk than a guy losing his yubikey? Depends on who you are and what you do. There are even more subtle costs like people forgeting their yubi at work/home and losing man hours for when they have to retrieve it. Malicious insiders swiping a fido key to do harm because of how much trust a session authenticated with a fido key has,etc...