Having a mechanic replace a vehicles serpentine belt can be between $60.00 to $200.00 including parts and labor.
If you replace the serpentine belt yourself then it would only cost your time and the cost of the serpentine belt.
Serpentine Belts range in cost of between $25.00 to $50.00 and sometimes as much as $80.00 to $100.00 depending on the make and model of vehicle.
The last time I had a shop replace a serpentine belt on a van that was hard to get to the serpentine belt on it cost me $120.00 including the belt and the labor costs of the replacing of the serpentine belt.
Some serpentine belts are easier to change out than others and if it's not too difficult then I'll change them myself but if the serpentine belt is harder to get too and change then I let a shop do the job and it's well worth the cost.
I usually only need to change the serpentine belt every 5 to 10 years or after 100,000 miles or more and I will always replace a serpentine belt on my vehicle if it's been longer than 5 years and I plan to go on a long road trip.
I hate being broken down and when a serpentine belt goes bad and breaks then you lose the power steering, water pump cooling etc.
So your engine can be damaged as a result of the serpentine belt breaking and also you'll be stranded unless someone is near to help you.