I've got 215,000 miles on my E36 and I have to top off the coolant maybe once every couple months, but that's mostly just in the winter months. Basically no more than one cup's worth. Been doing this for many years and i have no clue where it goes. No external leaks and no signs of a bad head gasket (I've done used oil anaylsis'). I now just consider it normal.
If you want things done right, you gotta do it yourself.