If you are storing good within the EU, you have to report all movement of products (warehouse to warehouse, supplies to warehouse) with the corresponding cost price. Goods movement values help authorities to understand the value of their cross border trade.
In simple terms, the cost price is the total purchase price plus everything you payed to have them in your desired warehouse, ready to be sold, not including any duties or taxes.
+ Purchase costs net
+ proportionally shipping costs to arrival destination (no custom duties, no Import VAT)
+ Packaging and inspection net
= Cost Price
A detailed description can be found here: Wikipedia - Cost Price
The cost prices are usually not lodged at Amazon; that is why you need to enter them to our yourself - at least only once (in case you want to upload many product at once read more here How to bulk import products).