No CPO.....I'd start at 50k. CPO is pretty cheap for the dealer. I have a very decently optioned 550xi sport (non msport) with only 3800 miles on it pristine. No smoke, no stories, just a perfect car and no way I'd let it go for cheap. Even trade in for my car dealer trade in should be in the 56-57--which is nuts. An equivalent 550xi 2013 with my options is going towards 80k.
They can throw in the CPO.
BTW, I traded 1 year old pristine 2011 535xi with under 3k on it and the dealer certified it. You run into problems when BMW discounts a new car so much vs. a used car even with new models. Push against the dealer and have them throw in the warranty. With the year end (really, during most of the year you can get good deals) I don't see the logic in buying used. You're still paying big bucks for a car. It's not in the 10-15k shopping segment. If you're will got go for the big buck, 55 tp 65k shouldn't be that big of a hump to jump.
In the leasing game, it makes no sense. You go for the newer model with the better options.