Depends on what the shop actually does.

To replace the spoke and retrue the wheel is about a 10 minute job so I'd estimate it should cost around $10.00 plus the cost of the spoke. The problem with that is that it merely returns your wheel to what it was previously. Unfortunately, what it was previously, was a wheel that's about to break a spoke.

What I prefer to do is to check and equalize the tension on all of the spokes before retruing the wheel. That's, obviously, going to take longer so it's logically going to cost more.

Incidentally, I used to try to keep a complete spoke inventory. No more. There's way too many different ones and they don't move fast enough to justify maintaining a complete stock.