A costing method by which the value of a pool of assets or expenses is assumed to be equal to the average cost of the assets or expenses in the pool. For example, if one share of Company A's stock is purchased on June 1 for $50.00, again on June 15 for $35.00, and again on Aug 10 for $40.00, the average-cost method assumes that three stocks were purchased for an average cost of $41.67. This number is arrived at by adding $50.00 + $35.00 + $40.00 and dividing the sum by 3, because there are three stocks in the pool.