If I as a SaaS provider get my SSO SAML integration via a provider like Okta or Auth0, the auth provider pricing itself is also on a "call us" tier, with a per-federation pricing in the low four figures for each individual company connecting to me via SAML.
It's pretty insane, so I'll state it again: To have a company connect to my SaaS via SAML, I as the SaaS provider have to pay my auth provider $X,000 per year for the privilege. Not counting the base enterprise tier pricing for the auth solution itself. So then I have to roll my own solution if I want to provide it for free, and I get the joy of supporting the long tail of broken SAML implementations on both the service and identity provider sides. For free. In a perfect world SSO wouldn't be a shitshow and everyone could have it for free, unfortunately that is not this world.