determine the mean cost per unit I have N orders. Each order consist of x units and I know the total cost of each order. If I would like to determine the mean cost per unit do I use approach 1 or 2 listed as follows:
(1) Determine total number of orders X and total cost of all orders Y. Then divide Y by X to determine the cost per unit.
(2) For each order divide the order cost by the number of units of the order resulting in X1. Then take the mean of X1.
 A: Only the first calculation gives you the mean cost per unit. 
The second gives you the the mean unit cost per order. 
That is a completely different question.
Let me explain this further. Suppose you buy lettuce for a grocery chain. You have always had two suppliers from which you bought equal quantities, but now you want to keep only one in the future: the cheapest one. You can expect the price AND consumption of lettuces to vary next year as it did last year, so seasonal variations will play the same roll in the future. Then you need the the average price per lettuce, i.e. your first calculation.
On the other hand, suppose you plan opening a furniture studio that will need a tons of wood per day and you want to forecast what you will pay on wood. Suppose in the past you accidentally bought some wood, sometimes more sometimes less, and you know there were some seasonal differences in the price. Now you are not interested in the average price you have paid. If you were lucky to have bought your largest quantity when wood was cheep, this should not bring your estimate down. So now you average the unit price over the orders. That's your second approach.
