You realize that the spoke needs to be the correct length, right? And that not only is there not a standard spoke length, but usually the drive side spokes are a different length than the non-drive side spokes right? And being a millimeter or two off is consequential for the wheel, right?

First the bike shop needs to accurately determine the correct spoke length (the broken spoke is broken, so you can't really use it to determine the length accurately). Then the correct length spoke needs to be either found or cut (a spoke cutting machine is quick and easy, but costs $4k to buy, keeping spokes in stock in 1mm increments is both expensive and impractical given the range of rim/hub/lacing/wheel dish combinations).

Then, after all that is sorted, the new spoke is laced in, and the whole wheel needs to be re-trued and re-tensioned. Remember, the reason a spoked wheel works so well is because it works as a whole. When a spoke breaks, this will effect all the neighboring spokes as well. So it is not as simple as slapping on a new spoke and tensioning that one spoke to how it was before.