Not sure if you are a BMW noob, but you must also consider the cost to maintain factor. If you plan on holding on to a car past 50K then you should familiarize yourself with the expected maint costs.
M are often time prohibitively expensive to perform maintenance on. Often time 30 to 50% more for the same service as a non-m car. Add to that the M mark-up on top of the BMW mark up on aftermarket parts prices.
Gas prices are also a consideration, they wont get any cheaper. Insurance is higher also.
That said, owning an M should not be about being "money wise". It's about being a connoisseur of fine cars. The M is a work of art. How do you put a price on that?
In any case, owning a car is much more then the sticker price.