We're not going to put a number here that doesn't hold up once someone's opener, door weight, or spring configuration turns out different from average - that's the kind of promise that looks good on a page and falls apart on the phone. What drives the cost is spring size and count (single vs. double torsion, or an extension setup with cables), whether the drums or bearings need replacing alongside the spring, and whether the door is a standard single or a heavier double-wide.
We'll tell you the real number after a look at the door, not before. If a spring fails and takes a cable or a bent track with it, we say so on the spot instead of discovering it mid-repair and changing the price. That's the honest limitation on this page: we won't quote sight-unseen, because a wrong quote costs you more trust than a slower answer costs you time.