The cables on any garage door are gauged on two things. The weight of the door and the length of cable needed. The weight of the door will determine the gauge or thickness of the cable. And the length of the cable is determined by the type of spring system your garage door has. The two basic spring systems are torsion and extension springs. The torsion spring system uses a lot shorter cable just about a foot longer than the height of your garage door. Extension spring systems use about four times that length of cable.
This is because extension spring systems work on a pulley system and the cable has to run from the bottom of the door all the way up and to the back of the door and back to the the front of the door where it is tied off. Also there is or should be a safety cable that runs thru the center of the spring that is there so when the spring breaks the cable keeps the spring from flying thru the garage and causing any damage or injuries. So on a extension cable system you would need about 30 ft. of cable for each side so about 60 ft. total.
So the length of cable that your garage door requires will determine the cost. Cables are one of the least costly parts on a garage door. A set of 2 cables can cost around $20 retail. And a local company will charge around $75 to $100 labor plus the cost of cables to replace them. You should always replace both cables.