(My apologies for my poor english, this is not my native language)
I think that there are four differents ways to consider an openspace sim.
1) It's owner's privileged place. He meet there his friends for chatting or dancing and so on. There's no busines here, only few avies/day, generating no extra lag.
2) It still is owner's privileged place, but he got there his own shop and sales objects that he made by himself. This is a small business, generating small trafic and small lag.
3) It's used for RP. Traffic increases during melees and meetings, but this is for short rushes. Small private business (Life controller, few rentals, ...), medium traffic, generating peaks of lag.
4) The openspace is planned to be exploited. Shops and clubs are rented and their tenants sell there what they build or third person has builded. This is a full business place, generating a lot of trafic and lag.
Considering 4 points above, could be set up an openspace sim's control, based on the traffic. (Easy to make: it already appears on the finder's list). It's basic fee could then be indexed to the real sim's exploitation.