I've never been diving anywhere that charged for the weights. That said, there was a significant charge for any weights lost/not returned. This may no longer be the case because fear of that massive charge could cause a diver in trouble, to retain their weights rather than dump them.
Be what may, I agree that a shop getting that many tank rentals from you should be willing to negotiate.
Be what may, I agree that a shop getting that many tank rentals from you should be willing to negotiate.