The billing really has to be based upon your reason. If it is purely to lash out at the OW...then it is spiteful, controlling, and for all the wrong reasons.
If it is because your firm needs the cash flow and billing for server space is the usual course of action....then you should bill it. I would suppose a server runs like every computer...the more on it...the more maintenance it needs.....the greater liability of system crash and infections....so there is a cost of having empty space compared to an operating file system. So it really isn't free space...it is costing you.
The real key is YOUR REASON for the course of action.
