I took a trip during the summer of 2010 in my then sDrive Z4 35i 2009 and my average fuel consumption was around 8.5 to 9.5 L/100 KM. That was my average for the cross country trip and back home (Toronto - LA and back home to Toronto)
Here I was just after stopping on Route 66 to look at the biggest chair I ever saw
BTW. Fuel consumption should not be a concern when driving this kind of car, I keep an eye on it mainly to keep an eye on the engine healt and my driving style