Affiliate deals with content sites - which are more than just advertising because they tend to have insane revenue sharing deals for the lifetime of the lead - often 50% split of membership fees for the life of the subscription.

So from just a single user signing up for a $30/month subscription, you would make over $180/year.

My partner enjoys a healthy secondary income (way way above the monthly figure mentioned in the original question) from such activity.

If you do big-data analysis and/or curation of content you can probably sell this to other sites too as many in this space are cash rich and tech-ability poor.