compare a hsv senator to an m5 its not a bad comparison (big 4 doors, big engines, high level of equiptment) particularly talking retail prices. You'll lose alot more on the m5 in the long run too.
20L/100 duno about but if i flog my skyline its not exactly a hybrid either. Not to mention current models have AFM (switch off 4 cylinders and the LPG system that allows them to run on gas and keeep same performance)