I didnt pay. they gave me goodwill - i am still awaiting the paperwork from them to check they did actually change all 4 as they said. It took 2 days for the goodwill to come from bmw germany so my car was there for 4 days.
good luck with yours. however my concern is still whether it will happen again. they told me i wouldnt be coming back in with that problem again.
I did say i would lay down with a placard in front of the dealership rather than pay.