My experience, mudflaps are just put on a car to get plukked off when going onto driveways especially with those sliding gates that have a raised track! Looks good on the E46, but not so sure about them on the E90, and certainly not on an M sport. But that's up to personal taste. And as far as function, it only makes a very very small difference on the E46 compared to before I fitted them - car still gets filthy on the sides and I am sure that things thrown up by the wheels still get to the bodywork...
Not sure how the prices compare, but last time I had to replace a mudflap on the E46 I paid something like R100 for a pair (they sell in pairs), and that was either for the front or rear, can't remember. This was from Forsdicks Tygervalley.