Rear Suspension Coil Springs problems of the 2004 BMW X3

I have been alerted by my local mechanic a few years ago and in 2011 by my local BMW dealer that my left rear coil spring has broken due to rust. I have been told by the local BMW dealer that this is a common occurance for all Bmws that operate in the northeast due to salt. The dealer's memo to me states: "if not replaced, worn coil springs will not allow the suspension system to move and adapt to changing road irregularities". The BMW dealer wants $640. 24 to replace both rear springs. My vehicle now has 80,000+ miles on it and I cannot afford to have it fixed. The car has always been garaged, as has been our other vehicles, none of which that has had suspension problems due to salt/rust. One vehicle has 280,000 miles on it. This is a major design/performance flaw that may have bad consequences for BMW owners.

A piece of the strut coil spring fell off while my car was parked. It appears to be rusted and just cracked off. I returned to the dealership showing them the piece I found and they advised that this is a normal occurrence and it will cost me over $500. 00 to have this repaired. I only have 57,000 miles on my car and I don't feel that I should have to pay for this as the coil appears to be defective.