Having bled brakes quite a few times and most recently on my M last weekend, snapping 2 bleed nipples is a pretty poor show, made even worse by an unwillingness to fix the problem. If you've snapped one you should be taking extra care on the next. Giving them the benefit of the doubt, someone might have over tightened the nipples on the previous brake fluid bleed. But then i assume its that dealer that had done the previous bleed so ultimatly they've caused the problem. Breaking two nipples and then expecting the client to fork out for new callipers, even at a discounted rate, is poor service in my opinion.
Shows that this particular BMW main dealer is only interested in replacing bits on newer cars and the 'technicians' clearly don't have the skills to deal with slightly older models or anything more than basic replacement work. Hence once the car is out of warranty a proper specialist service history has more value than lots of main dealer stamps.