This web site has a search engine that will give you the average cost according to the area that you live in. I have found it to be reliable....only thing it does not add the cost if the water pump should that be replaced at same time.
I reserched maintenance shecdules and cannot see any reccomended interval for the timing belt on your car. Some manufactures do not have a recommended time to replace. In that case it may not be needed until that one day when the water pump should start leaking and then that would be the time to replace it.