£140ish is the price now. They used to be £0ish, but BMW had to have some more made to the original pattern and obviously didn't have the economy of scale from when they were originally made.
The only alternative that I know of is non-OEM.
If it were me, I'd try to negotiate as good a deal as I could from the dealer, bite the bullet and cough up, but I understand why most wouldn't.
Another alternative would be a salvage diff cover, or even a salvage diff as long as the bush is in good condition.