I can't actually think of a practical use case for the Pro-xx accounts. A fairly simple blog would easily have a 100 assets or more. Even a simple corporate blog would have 100 assets after a few months.
In particular, I'm trying to figure out what your expected use case is for the Pro-20 account ... what kind of site would I have only 20 assets that I want multiple editors to be able to change?
Websites tend to be many asset/many editors or many assets/few editors. I can't think of too many few assets/many editor situations.