I recently picked up a 2008 528i CPO. Approx 40K miles, $30K USD. I considered it a fair value, not exceptional.
Personally, I liked the idea of having the CPO for the extended warranty on major items. Of course, I won't know how good it is until I have to use it.
For $2600, the dealer offered an extended service package to cover the standard service costs for 2 years or 50K miles after the orignal factory warranty period expired. I opted not to do this. I did not see the value.
As for maintenance costs, I previously had a VW Touareg which was on the pricey side relative to maintenance. I expect the maintenance costs to be similar ~$1,500 per year on average given driving 12K miles per year.
I have always used the dealership to service my cars with the belief that they see them every day so they should know more and the fact that they may have info from the factory that independents may not have. Also, for the last couple of cars I have owned, the dealer provides loaner cars at no cost.
I would consider a well-respected independent if the dealer fails me in some way.
Overall, I have been happy with my 528i CPO. However, I have encountered a couple of quirky things that you may want to reserach and consider....1) An A/C odor caused by mold/mildew is considered normal. You will have to pay to have the system cleaned; 2) The bluetooth may or may not work for you. Based on my experience, if you do not have a phone on the "tested phone" list, bluetooth support will be nil from the dealer and BMW North America.
Other than these two things, I am a happy camper.
I hope this was helpful and timely.