Paid Memberships Pro has a somewhat unique monetization method for their plugin. They largely monetize by selling support and custom work, versus selling any kind of extension or functionality. All Paid Memberships Pro features are free.
A few years ago, this wouldn't be out of the norm, but today the primary model for success appears to be freemium or commercial only access.
Jason Coleman has published their revenues, broken down to both support and service components, since 2012. Their total plugin revenue has gone from $9,800 in 2012 to $127,000 (estimated) for 2015. The plugin totals have pretty directly replaced service revenues, which went from $157,000+ in 2012 to an expected $63,000 in 2015.
Jason breaks down the revenues with a year by year summary that's fascinating. For example, from 2013:
By the end of 2013, we had stopped being a WordPress business and turned into a Paid Memberships Pro business. Over 80% of our consulting business was doing work on top of Paid Memberships Pro.
And he thinks they have a great deal of room to grow. They have plans for new products for 2015 to help increase revenues beyond their standard $97 membership plan.
A final metric I enjoyed: A little over 6% of their registered website users have converted into the $97 membership/support plan for them at some point over the last four years. Their registered users on their site is almost the same as their active installs, as an additional anecdote.
An in-depth revenue report like this can be really informative, and I appreciate Jason opening up like he did. PMPro seems to be a nice lifestyle business for his family, and has a good bit of potential for the future.