This morning the Lab has announced the prim limit increase for ALL regions, private regions too. See: More Prims for Everyone!
While private regions have been bumped up to 20k prims, the Lab says next week (#49) they will implement:
If 20,000 prims still isn’t enough, starting sometime next week, Private Region owners will have the option to upgrade to a total of 30,000 Land Impact limit – more details coming soon!
Word is there will be a charge for the addition prims above 20k. Until the Lab releases the feature and tells us we are all speculating on the cost. Even what the Lindens have told us it is likely to be is not yet cast in concrete.