Complex pricing procedure

Hi Gurus

My client has a requirement whereby they need to bill on materials which have over stayed in their premises in excess of 30 days. The client deals with the business of storage of materials for it customers. They have proposed two levels of tariff 30-60 days 20USD and 61-90 30 USD. the rates are to be prorated i.e per day. Meaning that if a customer has 10 boxes in store which have over stayed by 20 days he will pay ( 20days X 10 boxes X 20/30). how do we go about this?

__.____._