I like Patio11's characterization[0]:
> The right way to think of the "SSO tax" (where companies charge extra for security features) is "You are being offered a dual use product backed by a strong engineering team for far less than it would otherwise cost, with sophisticated enterprises picking up the slack."
That said, TLS/SSL used to be the preserve of the enterprise too (or at least the ecommerce site).
There are lots of free options, including 3rd party servers and libraries. I'm hoping eventually SSO will be, if not in free versions, at least not isolated to enterprise plans.