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). Most springs have a 15,000 cycle lifespan. 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. With this in mind, we encourage people to replace both at the same time, that way you are not paying more to have a repairman out twice. Also, old springs are dangerous for a repairman to work with, as they are very likely to break.
For a typical spring repair, you can expect to spend anywhere from $250-$400 to have new springs installed.