Cost of sales is sometimes called cost of goods sold, but I am sure that you can cope with this complex variation in nomenclature. Cost of sales is, how can I put this simply, the direct cost of what you sell in any one accounting period.
For a re-seller, the cost of sales is the price paid to acquire the goods that are to be re-sold.
In manufacturing, the cost of sales is raw materials and other supplies consumed in making the product, plus directly attributed production wages and factory overheads such as heat and light.
For a software company, the cost of sales might be determined as one-hundredth of the original R&D cost for each unit sold (if lifetime sales are expected to be 100 copies of a program).
It makes sense to run through ...