OK, So I understand that you are trying to keep track of active item costs. If you want to put the cost of an item in before it sells you can do it in the product master tab.
The items on the Product Master tab tie to the sold list by the "ebay item description" field.
So for example, if you sell "New 1000 pack of widgets"
you would put that in as the item description in the product master, and fill in the purchase costs, and they will transfer to the sold list when they sell.
You can also use the button to refresh items that have already sold, in case you add the information to the Product Master tab after the item has already sold.
Ill be sure to make an animated tutorial for this to go in the tutorial section as well, thanks for the question!