Our approach has been to charge a certain amount ($.0015) per month for
each file stored and a certain amount for amount of data transferred.  We
do not have a monthly charge for amount of data stored.  The transfer
charge is 3 tiered: $.15/mb up to 50 mb per node per month, $.07/mb for
next 150 mb and $.01 per mb above that.  For large amounts of data we
charge $25 for the 1st Gbyte and $10 per Gbyte transferred thereafter.