The dealer always wants an arm and leg for even the smallest thing. Why don't you try taking it to another place that is not the dealer, but that is familiar with Mercedes Benz. Here in Miami the prices for service at the dealer are ridiculous, so I try take it to other places that I know and that can give the same service. Specially when it is something that seems to be simple. Good luck and let me know if changing the bushing helped to get rid of the sound.