Thanks all for your responses.
I guess it isn't the most scientific way of figuring a Bimmer's rate of depreciation. May I rephrase the question:
Who knows what the average depreciation is for a 3 series sedan w/ M Sport. Do they depreciate at the same rate as a normal vehicle, say 30% after 3 years (average km and good condition etc) or do they hold their value a bit better. Any references would great.
Redbook is crap. There's very little correlation between real world prices and what they list there.