On average I would think a muffler shop would charge $40-60 for labor for just swapping a muffler.
I found that you can save a decent amount of money by showing up at a muffler shop that isn't a name brand (one of those dirtier/greasier looking places) right when it opens up (so they aren't busy yet), don't call in advance, and show up with cash in hand (bring 5's/10's so you don't have to get change). I had a lot of stuff done on my truck for considerably lower than phone quotes by going that route.