The cost of a rear drum brake job can vary depending on several factors, such as the make and model of the vehicle, the quality of the parts used, and the labor costs of the mechanic. On average, however, a rear drum brake job can cost between $200 and $400, with the majority of the cost being the labor. Some mechanics may also charge extra for additional services, such as cleaning and adjusting the brake components or replacing the drums and shoes. It is always recommended to shop around and get quotes from multiple mechanics to ensure you are getting the best price for the job.