I would be less concerned about finding a CPO as I would finding the right car for you that has a good history. CPO cars will demand a premium mainly because they are sold at BMW dealership, not necessarily because they have CPO obviously this adds a premium though. A solid PPI would check the same type of thing the BMW dealership will check for CPO, you can always buy an extended warranty later. There are deals to be had out there, just have to look very hard. I was in the market for a while, and was tempted to "settle" because of a deal. Very glad I didn't. I think 09's with lower milage are not worth 50k+.
There is a very nice e90 for sale at enthusiast auto that is well below kbb. May not be the right one for you, but to me it signals that the market is dropping faster than many would like to admit. It does not have CPO, but like I said I wouldn't base my decision on whether is has a CPO in this market.