Agree with TW Fox's comments.
Slightly tangential, but on an FB group this morning someone was asking about the E90 318i. An owner posted a rough average of 30MPG. An N55 335i will return at least 25MPG (often up to and beyond 35 in my personal experience, but I did a fair bit of motorway). Yes a 320d is a different matter (and a fair comparison would be a 330d), but I bet that 318i wouldn't be returning more than low 40s at constant 80-85MPH.
I guess the point is, these aren't light cars, E90, E60, F10 (especially) and a four cylinder motor needs to be 'on the boil' a lot more.
But from a company car taxation, depreciation, initial purchase price perspective, or a private buyer's 'whole cost life' basis, the 18i or 20d will always be more attractive. As far as maintenance costs go, there isn't really much of a different between any of the cooking models, it is only on the M cars that you see a step change and a lot of that is down to market pricing rather than the cost of the components.
Run a price for an oil change and microfilter for a F10 550i and F10 M5 on the BMW Service website. The M5 is almost double; the cars both use the same oil filter and the same microfilters! The oil in the M5 needs to meet LL04, same as the 550i...