IMO the biggest difference between the two is self-hosting which makes sense for the price difference.

Even setting that aside, SSO is a feature which is very meaningful to businesses and relatively meaningless to individuals. Because of that its often used to differentiate between the customers. This differentiation results in individuals getting a discount at the expense of the businesses; which to me makes sense.

Ha! sso.tax, what a great site. As an IT person I always thought this same thing with SSO - even if you have an identity provider, it's often under utilized because nearly everything else needs to go to enterprise pricing for SAML auth. I wouldn't mind paying $1-2 more per user/platform, but as sso.tax tallies, the price jump is often much more.

On sso.tax, it states that "Single sign-on (SSO) is a mechanism for outsourcing the authentication for your website (or other product) to a third party identity provider, such as Google, Facebook, Okta, PingFederate, etc."

Isn't this the definition of Federation, rather than SSO?

As I understand it, federation enables two separate instances of some particular service to interact. They can still use single sign-on independently for their own authentication needs.

I get where you are comming from, but from sales perspective, charging extra for SSO makes total sense.