The reason there's a belief of an SSO tax is not that you have to pay more to get SSO, it's that companies use SSO to get you to pay for features no-one ever wanted but they did a lot of development on because they know you need SSO.
And the gatekeeping of it behind sales people.
I’ve been around product development for 20+ years and I have NEVER seen a feature that “no-one ever wanted” get a lot of development resources.
What I see a lot are features that a few huge customers want get those resources, and other people without that perspective write them off as useless. Meanwhile we’re closing 9 figure deals on the backs of those “useless” features.