We're not going to put a number here that doesn't hold up once a tech is standing in your driveway looking at the actual door, spring size, and opener model - too many variables change the number for that to be honest. What we can tell you is what moves the price: whether it's one spring or a matched pair, whether the drums or cables need replacing along with the spring, and whether the opener itself is original to the house or a later swap.
Here's the honest part. On a door with 1960s or 70s hardware - common across this stock given the 1953-1995 build range - sometimes the track, hinges, and drums are worn enough that patching just the broken piece means you're back out here in a year for the next failure. When that's the case, we'll tell you plainly that a full hardware replacement, not a single-part repair, is the better use of your money. We'd rather lose the smaller ticket than sell you a repair that doesn't hold.
One limitation worth saying straight: if a part is genuinely obsolete - an odd-sized drum or a discontinued opener board from an original 1960s install - sourcing it can take longer than the emergency itself, and replacement ends up faster than waiting on a part nobody stocks.