So, to me, if the belt is bad it should have been replaced when the alternator was replaced. Here's my thing these days. I take my newer cars which both are less than 4 years old and both have low mileage to the dealership b/c I'm building my service history w/the dealership and that rapport. So, in case something big breaks, just out of warranty (in the past) I've found that that service history loyalty carries some weight in the form of not paying full price for a big part failure. So then if I take the car to dealership, I fully expect the dealership will charge me for everything from the air in the tires to the shop rags and (to me) I always fully expect the dealership to say "hey, we think you need a new serpentine belt, we'd like approval for that, we'll just charge you the $83 for the belt, no labor cost since we're already in there, etc." Same with a tensioner or pulley or anything else they find while they're in there. My point being, if it's a big dealership in this situation, I'm expecting to haggle a bit about this because I'm paying so much more in labor cost than I would at a small, independent garage down the street. So, at the dealership I would expect to pay $1,000 to $1,500 for an alternator replacement b/c they're going to do the alternator, probably the belt, probably the belt tensioner and I would expect they would suggest a pulley replacement "while they're in there" type of thing plus fluids or whatever else.
I take my older car to an independent mechanic, he tends to be more honest and sincere. If I took my old car to him, I'm certain the whole job would be about half (so say $500 to $750 if the dealership wanted $1,000 - $1,500) and if the belt was squeaky, I would offer to pay for both the part and labor because I know him pretty well and if he didn't replace the belt, pulley or tensioner in the first place, it was because he probably felt the part was still okay plus the cost for him to do the work is about 1/2 of what I would pay at the dealership.
