For 2009, the residual is what you noted at 15K. I haven't seen the 2010 numbers yet.

You can add 2% for 12K and 3% for 10K miles.

Your calculation is off though (sorry). Prepaying saves the interest on the depreciation amount, but you still pay interest on the residual amount.

BMW should offer a different money factor for a prepay. The current 2009 money factor is 00250 (6%) and I'd guess the prepay is around .00170 (4.08%).

So I'm guessing your prepay would be around $31,800 (includes 6% tax)