If I would be in your situation I would proceed as follows since
A) you already made 20 payments from your hard earned money and
B) your desire is to get a e92 M car before production ends.
So here is what I would do if I was in your shoes ......
I would deal directly with the dealership where I purchased the e90. Let them go and look at your car and get there price as a trade in (in the case at hand what the cash return on the 25K down payment and the 20 payments less depreciation would be). Then work out with them what the new e92 M should have then evaluate the situation.
I would not only deal with the original dealership where I purchased the car I would also find another dealership near by and inishiate the same request.
Also I would let both dealerships know that I am not only looking at them for a final price ... This will make sure you get a good price. In the end you are looking at 2 results before you.
Remember your now almost new e90 M is in high demand since production has ended for now of the Sedan.
Good luck let us know how you make out.
Last edited by Mr. ///M3 RD; 01-03-2013 at 11:32 AM.
Reason: Minor addition made