I know this thread is pretty old now but i thought id give anyone who cares an update.

BMW advised this was not a recall item for the e53 so it was not covered.
The brake servo / booster was about $1800 from an BWM dealer in Australia.
There was a wrecker on the east cost of Australia which would sell me a second hand unit for about $1000 delivered to Perth.
I managed to source a used part from the US for under $200 delivered.

I managed to remove my brake servo, reinstall the new unit, bleed all the brakes with new fluid and have everything back together again within a day. All up it cost me under $200 for the servo and under $50 for dot 5 fluid.

The brakes are now back to how they used to be and stopping strong