I wouldn't include that in enterprise, since you are creating difficulties for small companies to onboard with you. There are some companies that include SSO in the initial plans, like Zendesk, and they get us to happily pay for the plans that fit our scale. I'm pretty sure you will have other awesome features to include in the enterprise plan.

Please, reconsider and good luck! :)

Forgot to add: I'd prefer to pay for a cheaper plan that offers SSO than to self-host it "for free" and not have SSO.

Agreed. SSO is what would allow a bit of organic use in many organizations and create the demand needed to get a business to spend.