The discount you achieve on any second hand premium car will depend totally on how long the dealer has it and the demand for the vehicle type you are looking at the fact that you are not financing is no advantage to you as the dealer will get commission from the lender also the dealers wiggle room will depend on what he took the trade-in against and the amount of extras that were added to it

I've been in few BMW dealers recently and ther second hand stock was presented far from showroom condition!! poor detailing,cars with marks and some needing tyres there approach seemed to be less in the way of discount but replace or repair and defects there banking on the 2 year warranty selling the cars for them
If the car your after needs nothing then start low u can always go up!!