I have a 2000 ML430 with 112,000 miles on it now... The front pads need to be changed every 25,000 miles or so, and the rear every 20,000 miles or so. I drive a mix of city and highway... But I order my stuff through different places, the fronts rotors are about $70 per piece, the rear rotors are about $85 per piece, the front set of pads about $70 for the set, and the rear set of pads about $80.
So, total around $460 for complete change of everything... It's not too bad... rotors changed every 3rd pad change for front, and every 4th pad change for rear... no pulsating brakes and stopping great!!!