D'oh
Its seem just a little too obvious that if not everyone joins then the contribution will need to be re-calculated.
BUT you have to remember that if, for instance, S Yorkshire don't join the national scheme - then the costs of South Yorks will fall just to South Yorkshire - and not be added to the national bill. So the 'members' making the contributions falls by one, but the overall total the remaining members have to raise is £1.67m less (according to this thread).
So really it depends on which police authority you are talking about but, it seems to me, that for SY the remaining 40 or so members should make a saving.
I love this thread, as much as I want to stay away it gives me some form of weird S_M pleasure; just off to flaggelate myself.