Is the cost that can be assigned a "per card" value or a "total for the quantity" indicated?
When you buy multiple boxes of cards you are bound to acquire duplicates. Is the cost assigned for each item in the collection a unit value or is it a total for the quantity? I added 1 card and it's cost and then found a second one and only incremented the quantity field, should I have also added it's cost to the entry too?
The cost should be your total cost for all of the items regardless of the quantity. So you should increment the cost if you bought another copy and it cost you something. We do that because the cost can be different for each copy you buy.