It is hard to say without knowing the make and model of your car. Each one is different.
I just looked at our latest receipts because I recall replacing rear brake pads and sensors within the year on hubby and daughter's cars.
Last December, we replaced rear brake pads on daughter's car '07 Mercedes CLK 350 and it was $215 (this amount included a discount for multiple cars as well as a service discount that is routinely offered)
Last July, we replaced rear brake pads and sensor on hubby's car, '10 Mercedes E350 and those were about $250. (Again, this amount included a discount for multiple cars as well as a service discount that is routinely offered)
I have not yet replaced brakes on my car, SL 550 but I would expect mine to be pricier due to the sport car package I have. I would get the regular discounts applied as well.
Still... I trust ONLY our dealer to work on our cars.... this includes inspections, tires, you name it. This way, they are the only ones who have all of the records for the car and things stay consistant just in case there is a bigger problem down the road.
Also, our dealer's bottom line is repeat customers and word of mouth advertising for the company vs Joe Schmoo down the street who sometimes needs to amp up some profits to make ends meet for the month.
If I were you, I would get a second opinion just to be sure and have each mechanic explain everything they will be doing.