It sounds like a lot...not sure if they are packed every day and renting tanks every day? Then they would consider that normal revenue like how you mentioned it. You could always call back and ask if they have a package deal for cash?
In my rather limited experience, rental tanks go for ~$10-15 per day including the initial fill. They often give you a "travel day" on BOTH sides because it's hard time-wise to rent a tank in the morning, dive it, and return it by the end of a business day. So you pick up the tank on Friday and drop it off Sunday for ~$10.