I am not sure about the X5M, but the M5 I owned was eating a lot of gas in the city 22.3 L/100km.
I believe the X5M is going to eat more than that, if you can effort 60$ ( that was on my M5) everyday than have fun
I am sure you are going to spend more
Personally I would not suggest you to take it if you drive a lot, but " if you go M, you never come back again, there is no one to blame "
enjoy the power !