Duo SSO and Azure AD subscription levels

TL;DR: What Azure AD subscription level do we need?

We’re migrating from our Duo Access Gateway to Duo SSO and ideally we’d prefer to use Azure AD as our Idp. The only issue is cost. We currently only use the free version of Azure AD and with the number of users we have, upgrading would get expensive.

Thanks in advance.

According to this Microsoft Azure Active Directory plan comparison page all plans offer “Federated authentication (Active Directory Federation Services or federation with other identity providers)”, including Free.