Mercedes Benz repair: Brakes, changing brake pads, light rust

Question
I have a 2005 E500 4matic.  It has 23K miles, very rarely driven.  Noticing some light rust lines on the rotars.  What is the suggested mileage for changing brake pads or other brake service for this car?  Also any ideal what the cost range should be, hard to trust dealers.

Answer
well brake pads are generaally a little above 5/8 of an inch thick. if you get down to about a quarter of an inch it would be suggested you get your brakes done..As far as cost I wouldnt take it to a dealer or a carx..find an indepedant who works on mercedes I did a brake job all 4 brakes a secondary air valve and replaced 2 tensioners for 180 dollars took me 6 hours ..so go figure..MB and BMW are the easiest brakes to replace so if your paying more than 200 in labor your getting ripped off. Anytime you go to the dealer no matter what it is expect the bill to be a grand