I've always done all my maintenance work on my R1150RT
Brakes, ABS flush and fill, alternator belts, Wheel circuits, TBS, valve adjustments (Which all I've ever done is checked the clearance - they haven't budged since 12,000 miles) etc, etc. I now have over 190k on the old girl and had one problem - a stick coil went bad.
BMW dealers have all raise their prices and I won't go to one for any service work. Too many forums out there and they will help you solve problems.
It's just a motorcycle.
Some say the parts are ridiculously priced and some parts are. But... so are the parts for any bike.
Do NOT fry your alternator with too many goodies connected and running. A new one for an ST1300 is almost $600. (I can get a refurb for my RT for $189.00)
My RT puts out 700watts - the only thing more generous is a Gold Wing.
There are a ton of aftermarket BMW part vendors that can save you big $$. Beemerboneyard, EuroMotoElectrics, etc.
Let's face it - they all cost more than they're worth. It's recreation and anything to do with recreation will get into your wallet.
You should price some parts for a travel trailer or fifth wheel - SHEESH!! (Don't ask me how I know that...)