As someone who has worked with SAML and other authn/authz technologies, I can only say that the reason for the SSO tax is because that stuff is unreasonably complicated and hard to make work. From things like Microsoft's half-baked proprietary version of SAML to the typical company's crappy in-house login system that was thrown together on top of a system built with security as an afterthought, doing SSO is never simple. Security as an afterthought is always way more difficult than if someone had thought about user roles and permission from the beginning. On top of that, even little companies expect to have things like "well, the execs never really log in, so we need to be able to delegate permissions, but only for things the execs want them to do. We can't give them permissions to sign off on bonuses for themselves".

Knowing what I know, I don't really begrudge any SSO provider their premium pricing.