The only major maintenance items I've had to do was replace my tires multiple times. I have a 2001 330i sports package and after ~50K miles, I've replaced two sets of rear, and 1 set of front tires.
If you don't drive hard, and if you have all season tires, I imagine you'd have even lesser maintenance than mine.
Other than that, I haven't encountered anything else that costs too much to fix. It's been a flawless experience!