Where does the money actually come from? Other adult businesses advertising on your site?
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.
What are the needs and the problems that they are facing. How to identify the possible customers. Common affilate scheme in the industry, etc ...