Just reading through an old book of Z3 Road tests and the statement from BMW of the time that is much repeated throughout the book is apparently only 4% of people purchased a new Z3 with performance being the number one reason..The looks and open top was the highest in over 70%.
The M Z3's are a very specialist car..If you have a pyramid the 1.9's are at the bottom with the M's at the tip with the 2.0,s, 2.2s, 2.8.s & 3.0s inbetween (I think the coupe are maybe higher than the tip on reflection
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There are fewer potential purchasers for an M than say a 1.9 because of its higher value and what it is. Good luck with the sale but the reality is that if I wanted an M I'd have found yours for £18,000 by now and any car is only worth what somebody is prepared to pay for it..It maybe that if you hang on somebody new may come along that wants one and is prepared to apy the full asking price?