Most of the regular maintenance is comparable to most European luxury or high end cars (Audi, Porsche, MB). Figure $300 - $600 per year for the 15k or 20k service, maybe double that every few years for the "major" service intervals.
Transmissions, suspension bits, and power train components wear out on BMWs just like any other car. With anything over 60,000 and less than 120,000, assume that a car will either have the wear and tear pieces already fixed or else the new buyer should budget for it. Those costs are what they are - expensive. It is a must to get the records from the PO or just assume you will be on the hook.
I think the theory is that if you are choosing to buy a car that is higher on the expense curve (with the prestige factor) greater than a Hyundai, you ought to pay more to maintain it. When they work well, of course, they are awesome. My favorite car still is the 740i Sport that I traded in a while back. Any time something broke, it was costly, but that was mostly forgotten when you drove it.