Our campus shop does single spoke replacement for free...but you pay about $10 for a quick true.

Our near-campus shop does replacement/true for about $15.

Our not-near-campus shop does replacement/true for $20.

If you go to any good shop, fork out the $20 but expect that they'll show you...pretty much exactly what ptle above has said.

Very best way for your long-term cycling health? Either be shown once or try it once on your own, learn the process, and never need a shop to do that simple of a job again.

My $0.02