As of March 2015, the cost of a transmission flush varies between $100 - $140 depending on the make and model of the car. If the flush is being completed by a dealership's service center, car owners are expected to pay more than if done at an independent repair shop.
A flush is required at certain mileage intervals to keep the internals of the transmission operating smoothly. When transmission fluid is too old, it becomes thick and murky, resulting in hard shifting and even missed gears. To perform a proper flush, the car's transmission must be hooked up to a machine, which uses a vacuum to suck up the older fluid, so the newer fluid can fill the housing without any containments.