One guide to use for the 2012/2013 value would be the residual rate difference BMW would use say on a 24 month vs 36 month lease. That's what you need as a discount to avoid "eating" the depreciation before the car has even left the dealer lot.
I just got a 2013 335i at $4000 off the window sticker price. Then add 289 dealer fees.
Gone but not forgotten: 2008 BMW135i Vert
Gone but not forgotten: 2011 MB C63 AMG
Gone but not forgotten: 2011 BMW z4 3.5is
Added: 2015 Z4 3.5i