I think some of what you are seeing is a seasonal issue, i.e. Sept is traditionally the month when convertible sales fall off. Here are some sales numbers to consider (by the way, the 100 sales number for Z4's is a North America number only.....as are all the numbers below):

Sept 2010: 100
Sept 2009: 325

August 2010: 1,201
August 2009: 547

Hope this helps....

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2009 Z4 35i Titanium Silver
2011 135i Space Gray (Sept. '10 ED)
2011 535i Jet Black