how much can one expect to pay for rear brakes by a Mercedes mechanic? Also, is it acceptable to have to change the rear wheel pads when the car has 40K miles?

It will depend a LOT on what your Mercedes-Benz dealer charges per hour for labor. Here in PA, you are looking at $120-150 per hour at an MB dealer plus the parts. Rear pads with a light resurface of the rotors usually pays about a 2.0 per axle, so you'd be looking at $240-300 in labor alone. Rear pads typically do not wear out as quickly as front pads. In a typical braking system the front brakes are doing 70% of the stopping with the rear brakes taking up the remaining 30%. If there is a problem, such as a stuck E-brake cable or caliper, causing the rears to drag, they will wear out faster. Also, your personal driving style will affect brake pad longevity. Have you had the fronts replaced yet? If not, I would suspect a problem. 40,000 miles though is a good long time for most front brake pads. That's roughly 3-4 years of average driving (10-12k miles per year).