I bought bilstein b8 rear dampers for my C55.
100€ each (ordered in Luxembourg by internet).
A very good investment, previous ones were very used (110.000 km ... very stiff in normal condition, very loose when stressed), and the car's handling was quite wrong, especially under rainy conditions !
After on day of driving with the new ones, it is just fantastic (not as fun as previously but ... no regret) ... It seems I drive a new car !
Hope I will not regret to not have change front ones too