Sum of aggregate formula column?

Sum of aggregate formula column?

Hi!  I created an aggregate column to take quantity * cost.  The table however is not summing the total of individual items, rather it is multiplying the sum of items by the sum of cost.  Is there a different way to set this up?  

Quantity Ordered Cost Total (Aggregate formula)
Product 1 6 22.65 135.90
product 2 24 21.65 519.60
Product 3 42 2.25 94.50
Grand Summary: 72 47 3351.60