I have a box of new parts for my annual maintenance -- filters, oil, wiper blades -- that I plan to do in the next few months. The pile of parts cost roughly $200 if my memory serves me.
If you're referring to repair costs -- it is unlikely to be $100 or $200 each month. It's random, and there is a large dispersion about the mean. Many months it will be $0. Some months it will be $500, or $1000. If you're really lucky some month and need a motor and buy it from a BMW dealer, it might cost you $14,000 that month.
Consumer Reports generally gives BMWs black dots for "owner cost". I don't worry about it. Eurocars are an expensive hobby, but there are even more expensive hobbies.
2008 E91 station wagon 18K miles "Classic" BMW inline six, six-speed manual transmission, Alpine, Terra, Xenons, ZSP, ZCW, ZPP, PDC, CA, alarm, M shifter and M brake handle, Euro rear fog light switch, Euro aspheric mirrors, rear power outlets, Dunlop Winter Sports, Garmin nüvi custom cupholder mount, Lidatek, V-1 built-in remote display, tire pump, jack, floor mats, sand, dog hair, several pairs of smelly old running shoes.