My wife's 3.0 X3 gives around 13l/100km in general city commuting, dropping to 10-11l/100km open road driving depending on speed.
If you do get one, I recommend that you get the vanos seals replaced asap on that M54 motor, they are most certainly vrot by now and will make the car lazy and heavy on fuel. My wife's car gave 16l/100km before I fixed the vanos seals at 150000 km.