Yeah, even if it's a small price, it adds up and you don't have to bother onboard again. Once a client accepts the subscription payment (and is happy, of course), you get some money every month. If you scale up properly, it can be a decent income.
I am considering a PREMIUM subscription here as well, but I need to prepare the tutorials/books/private forums for it to be worth it. While it would be pretty cheap (probably something like $10/month), it would allow members to get more attention from my part (a bit of a personal mentoring) and access to information I have gathered from courses that cost me thousands of bucks.