Cost of replacing brake pads 09 ML550?
Your ML550 has bigger brakes but prices should not be much different. Akebono ceramic fronts were $75 and rear $50 (both included 2 new sensors) from RockAuto. If brakes do not currently pulse and rotors are thick enough then you don't have to replace the rotors. If you don't have them resurfaced ("turned" in some vernaculars) the brakes may feel soft the first few hundred miles until the new pads conform to the shape of the rotors. If you can feel and/or see the rotors are not flat then resurface or replace.
Both Mercedes and BMW (I had an X5) strongly recommend replacing rotors when you replace pads. The ML550 has vented discs all round; you cannot turn them, you can only replace them. The dealer typically charges around $1,000 for a complete brake job. You will save over $300 by doing it yourself, even more if you go with non-OEM parts. But be sure to use name brand parts such as Raybestos. A lot of auto parts stores sell very poor quality Chinese crap branded with the store's name; a friend of mine used some and within three months two rotors had warped.
The ML550 has one of the sweetest and most under-rated engines money can buy. It will throw that heavy truck down the road at frightening speeds. The last places you want to cheap out are tires and brakes!

Mercedes-Benz specifies 1) new rotor thickness, 2) service limit thickness, and 3) wear limit. "Service limit" means "don't mix with new parts" and "wear limit" means it is no longer safe. Prior to the wear limit the parts are perfectly safe.
Vented rotors means nothing as to whether they can be resurfaced/turned. The service limit means everything, the rotors can not be turned to less than the service limit and mounted with new pads because it is expected to be less than the wear limit by the time the pads next need to be replaced. 1986 VW Golf with solid rotors were so thin when new they couldn't be turned. Brake rotors on my motorcycles are even thinner. Replace motorcycle brake rotors every 10 to 20 sets of pads. Have worn new pads to the metal in less than 40 miles a couple of times now (racing in mud using Kevlar/carbon pads). Usually get 400 miles on the rears, 800-1000 front, in the woods.
You are correct. But the design engineers try to match parts, so if you don't replace the rotors there is a good chance that they will need replacing before the new pads wear out. It really boils down to how happy you are spending another Saturday afternoon under the vehicle doing what would have taken you an incremental half hour if you'd done the rotors with the pads in the first place.


