I would glaze over and probably be contacted by a divorce attorney shortly after an unlimited option started. My wife barely has patience now for what I am paying for !!!
Tony, you are right, unlimited leagues would have a cost factor if it was truly unlimited, so I am incorrect there. In my head and "unlimited" subscription would need to have some limits, the last thing anyone would want are people building teams and abandoning them if their AD card goes bad. The competitiveness would suffer for sure. A limit of "active" teams would need to be part of the subscription. But the ability to offer a variety of card sets wouldn't affect the cost model. The biggest challenge, if the application is designed well, would be processing time to run the leagues without buying more servers, not storage.
In my real world job, we scale servers pretty easily leveraging cloud providers. I know from hands on the cost of scaling server farms, what SOM would need wouldn't be a deal breaker from a cost side.
I could build an SOM cloud server today that could handle 50 managers/unlimited leagues/unlimited card sets for <$40/month.