Most cars today use a serpentine belt which usually drives all parts of the engine including the alternator.
Although there are some vehicles using 2 serpentine belts and if you're driving an older vehicle it will likely also have those V belts that are separate.
A V belt might only cost $20.00 to $30.00 while a serpentine belt ranges from $30.00 and all the way up to $150.00 depending on the vehicle.
The labor should cost from $50.00 to $100.00 depending on how hard it is to get the belt removed and replaced.
So in short replacing a serpentine belt for the alternator and engine will cost around $80.00 to $200.00 depending on the vehicle at least that's been my experience when replacing some peoples belts as some are easy to get too and some are much harder depending on the vehicle.