Check out Caspio Bridge approach to users licensing policy. The good news is Caspio Bridge doesn’t restrict the quantity of users (anonymous or authenticated). You can review the pricing here.
There is a "but" though..
On the other hand Caspio Bridge doesn’t have any build-in tools to work with authenticated users. Yes, there are some mechanisms to tune web user authentication and basic record level security but I think that won’t suffice for real-life app roll out, I mean with complex multi level access, not only to records, but fields as well.
Maybe everything can be programmed on the level of specific data pages, but my point is the flexibility of such a system will be limited, let alone the difficulties of its support.
My impression is Caspio Bridge is oriented on such apps: The main part is developed as a front end (it can be any listing, from simple list of the company news/announces to real estate listing with search capability, pictures and other stuff) and with one or several backends for different categories of users which manage this list. You can check it yourself taking a look at the ready-made applications.