How do I limit the amount of marketpress products to be displayed on a pro sites level?

Hi

On my multisite network, I'd like to be able to charge using pro sites something like this;

level 1 1-100 listings in their marketpress store
level 2 101-200 listings in their marketpress store
level 3 201-300 and so on...

Can anyone advise how this can be achieved please? I don't mean how many gb's of data they can store etc, I need to limit actual amounts of products able to be listed.

Thanks

Sally