I believe its the idiotic guesswork associated with fuel consumption.
Consider your most likely target market for that kind of car.
1. Young guy that wants a BMW, prob daddies money.
2. Daddy more then likely recons that the 328 will be too heavy on fuel, maintance, and everything else, and that it will be too quick, but a nice mild mannered 318 is a good bet.
3. Then you have the guys that wants the "is" cause they heard that it was cool...
Those factors = supply and demand... demand = price ?
my .05 cents worth