I've been charged varying amounts for spoke replacement & wheel truing. Between £5 & £20
The £20 was mid ride, 25 miles from home & didn't fancy risking the ride home, so was 'happy' to pay the £20. The next broken spoke I had was nearer home, so I limped home & then just took the rear wheel to a LBS, who only charged £5 for the repair
& IIRC the other time when I've needed it doing I was on the WOTR near Ripon, so stopped at Moonglu in Ripon, when I rung them to get their location they said they would have time to fix it, so I was just going to buy the spoke & fix it myself, but when I turned up at the shop with a loaded tourer, they must have taken pity on me, wheeled the bike in & they did the job for a tenner
I now have the confidence to do something like this myself, but just depends where you are & what tools you have with you. If I'm miles from home & unable, then I'll gladly pay £20, cheaper than a taxi home