A documented rant has made the rounds at https://sso.tax , which lists all vendors and their pricing of SSO.
As a small team we get constant requests to integrate with a customers SAML provider - eventually we just switched to https://workos.com/pricing We explain to our customers that we have a hard cost for the integration and we pass that cost to them directly. The SSO version of our product and our self signup product do the same thing the same way - it's the compliance or risk management requirement mandated by our customers that require that we sell it the way we do. In our case our SSO or Enterprise version is $125 more expensive than the self signup product. Our money is in the product itself not in the SSO.