We're not going to quote a number here that doesn't hold up once a tech is standing in front of the door. Spring cost depends on the door's weight, whether it needs one spring or two, and whether the cable drums or end bearings are worn enough to replace at the same time - which they often are on a door this age. What we can tell you is that doing both springs at once, even if only one has failed, is usually the better call on a door this old: the second one is close behind it on the same cycle count, and a second truck trip to fix what you could've done once costs more than the part itself.
For an actual number, you're better off asking during the call than guessing from a page. The pricing and full service details live on our main Richmond spring repair page, since that's where we quote by door, not by town.