A measure by Financial turnover sounds dangerous to me as that would only enthusiast chapters to count everything as money, while the most work is done by volunteers generally. I prefer to think in terms of volunteers and activities over money.
Good point. I can't think of a reliable way to measure activity, though... turnover, while far from ideal, is at least easy to measure. Some combination of measures would probably be good (every chapters gets 1 vote automatically plus an additional 1 for every X members and 1 for every $Y of turnover).