2000 Buick Lesabre CAM Shaft Sensor

My 2000 buick lesabre all of sudden started to hesitate while I was driving and my engine light came on so I drove it directly to an auto repair shop in fear that it might die out on me. After reviewing my car the shop informed me the cam sensor had to be replaced and it would cost $475.00 and take 3 hours. My question is this a difficult task and does it normally take that long and cost that much? The mechanic said that because of it's location it was going to that time.

Thank you,
Karen