The reason for the disparity in pricing has nothing to do with the "security features" that are offered with SSO and has everything to do with the increased support cost that come with it. The company I work for offers a local based authentication and SSO. The support costs around SSO are higher because oftentimes people do not know how to integrate this they don't understand the mechanisms of it and it takes more work and there's more questions and there's more fiddly bits to get right. When those fiddly bits break the customer themselves doesn't often understand it and are engaging support to not only understand their third party external SSO but also that integration with their underlying identity mechanisms as well.
So all things are not equal between the two and therefore they should not be equally priced.