If you do not change the default value for charge base/block, the cost will always be zero because the default base charge per block is zero.

http://www.cisco.com/univercd/cc/td/doc/product/voice/sw_ap_to/admin/admin_rp/1_1_1/user_gd/artch2.htm#1068744

You can use ART to set a base monetary rate for the cost of calls based on a time increment. Then, you can further qualify the cost by applying the time of day and voice quality factors. This feature is commonly used by service provider who must account for service to subscribers. Some organizations also use this information to establish billing costs for users and departments in the organization for accounting or budgeting purposes. Reports that use these rating parameters include individual bill, department bill, Top N by charge, and top N by duration, among others.