Managing cost prices is crucial to helping you monitor your shop profit and for knowing important values like how much your unsold inventory is worth at any given moment in time.
For speed and rapid processing of shop orders all cost prices for your inventory are stored in the Profiteer App itself. When you accept a subscription to the App all your sales history is downloaded to the App and all your inventory is setup in the App, with zero as a cost price for each variant.
Therefore the first task for any merchant installing the App is to set cost prices for all your variants. Once that is completed, you can rewrite the downloaded sales so that each variant with a zero recorded will have an actual cost price assigned instead. At that point, basic shop profit from sales is visible.