The first thing to understand is that the life span of a spring is based on the number of cycles (how many times the door opens and closes). A Torsion spring last 10,000 cycles. With this in mind, if you have two springs and only one is broken, the unbroken spring has just as many cycles on it so it is highly likely it will break very soon as well as being very dangerous for repairman because of the age and likelihood they will break. We encourage people to replace both at the same time, that way you are not paying more to have a repairman out twice. If you do have to replace your springs you can always upgrade to a higher cycle Spring.
For a typical spring repair, you can expect to spend anywhere from $250-$400 to have new springs installed.