Control arms are part of the suspension which locates and controls the movement of the hub and strut. There are usually a couple control arms on each corner of the vehicle. They also have bushings and ball joints which is usually why they need replacement ovetrtime, since those are wear items. The cost to replace varies depending on the vehicle. Control arms cost different amounts depending on the vehicle, as well as the labor to install. Some are easy to install and take little time, while others can take several hours.