If you have not already, login to the Control Center at https://www.webcontrolcenter.com/customer.aspx and from the menu item STATISTICS select SQL disk usage history.
To view the amount of space you are using in your SQL database you can use the SQL Disk Space found under the Statistics menu.
To use this option choose a month/database to view SQL disk usage history for that month. At the end of each month, the "average" usage is calculated for your site. If the result exceeds your plan allotment, a fee is assessed.