I'm certainly not an authority on this topic - so take what I say with all necessary caution - but I'll take a stab at answering. The bulk of my experience has been with very old (20+ years) and very new (0-3 years) BMWs. I recently bought a middle-aged (2001 model year), well-maintained and beautifully-kept E46 with lowish mileage and the ownership experience thus far has been instructive.
The vehicle I bought had 110,000 km at the time of purchase and I've put on around 7,000 km in the intervening months. Despite passing a PPI with flying colours, the car was in a maintenance-intensive mileage range at the time of purchase and costs have added up. Including scheduled maintenance, I've invested around $2,630 in various things. Hopefully these ownership costs are front-loaded and will balance out as the miles add up. It's important to attend to problems as they crop up instead of deferring until the annoyance/risk factor outweighs the grudge cost.
I think it's best to enter into the 'relationship' with the expectation that costs, including those associated with regular maintenance, may be radically higher than those associated with a Civic or Accord, but the daily payoff in driving satisfaction and general enjoyment of the car will be equivalently greater as well.
Some old BMWs