Given that MarkDavis has run away I guess I had better explain what I was on about. He won't be happy but there we go.
Generally speaking, the running costs of a BMW are dictated not by the size of the engine but by the cylidner configuration. The engines themselves within each cylinder configuration are virtually identical, differing usually only majorly by their displacement. In 3 Series 6 pots of this vintage, every 6 cylinder engine will be an M52 (Unless its post 2000 in which case M54) As a result, the following is usually true:
They all cost the same to repair - becuase the parts are the same. So if something breaks on a 323i, it costs the same to fix as if the same thing had broken on a 328i. Of course, the rest of the car is also the same, so if you need new suspension on your 323i, it'll be costing you the same as it would on a 328i.
They all deliver largely similar fuel consumption - this is the biggy. For the 5 Series range, for example, there is a difference of about 1.5mpg between the 520i and the 530i on the combined cycle. In other words, as near as makes no difference. Going for the smaller engined model does not deliver noticeable fuel savings.
In the case of models produced before March 2001, they all cost the same every year for Road Tax.
Infact, the only real appreciable difference in running costs is insurance. For everything else, the rule of thumb is simple - first chose how many cylinders you want (Becuase running costs DO vary between these), then simply chose the biggest engine of that cylinder configuration.
Otherwise you pay 328i running costs but don't get 328i benefits.
This applies at least to the models I've looked into - so 323i/328i, 320i/325i/330i, 523i/528i, 520i/525i/530i, 535i/540i.
Unless you can get a particularly good deal on a 323i or a 523i or whatever, don't bother becuase you save no money.
So there you go Mark. I stand beside my assertation that going for a 323i is a pointless excercise unless one jumps out at you at a bargain price. And not a 4 pot Mondeo in sight for my justification.