gametec12uk

Hi just a little advice if possible from anyone who could help. I have a 2001 voyager 2001 2.5 lx. Mileage is 76000 all main dealer servicing and belts done when required. Yesterday I broke down heard a loud clattering noise then power steering went and battery light came on. Pulled over and called the AA. The AA man said the tensioner was seized on the drive belt and required replacing so it was subsequently towed to a garage. My question is : Is this an expensive repair? The garage are looking at it and say that they are also taking the cam belt cover off as it looks like bits of the serpentine belt could have gone in that compartment. Does this sound possible or are they making a job of it. Any advice would be appreciated as I am completely thick about this sort of thing thanks.