The pricing of a cambelt change will vary between engines and manufacturers depending on the precise arrangement of the main belt and auxiliary belts and the exact process for each job. On the 3.0-litre engine the water pump is driven by an auxiliary belt and not the cambelt, so although changing the water pump will require the removal of some components for access, changing the timing belt is a more involved job which among other things will require the camshafts to be locked off to keep the timing correct, something which is not required when changing the water pump. If you are unhappy with the charge you could ring around other Jaguar independent specialists for alternative quotes.