ALL new cars depreciate quite quickly - the rule of thumb is that you lose between 25-30% as soon as you drive it off the forecourt. Yes there are a very few exceptions to this, Ferraris with long waiting lists, etc. but this applies to the vast majority of cars. Then the depreciation carries on each year.... New cars are never a logical financial decision!
So for a 2016 to be sitting around $75k is no big issue, particularly as since that car was bought there has been an LCI and a new model, the Competition, introduced. Both of which would have a negative affect on the value of existing cars.
What is it you are looking to get from your M2 ownership, fun from driving a great car, or a long-term financial investment? Unless you want to keep the car in your garage for 30+ years it is unlikely to be the latter i'm afraid.
My suggestion would be decide which model you want, do you want to wait a bit longer for the new Competition to drop in price, how much you want to spend and look to see what is the best one you can get for your money. Then enjoy the car and don't think about the costs.