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.