There’s now an extra option available for purchasing Azure Multi-Factor Authentication and it’s added to the January 2016 Product Terms document (page 53) as a user-licensed product available through Volume Licensing programs. The other options are to pay for it out of Azure Monetary Commitment on either a consumption basis per 10 authentications or on a per user basis with an unlimited number of authentications allowed per month. This page is a good resource: http://bit.ly/1JUVugq.