Generally, a product cost price is defined in the Prices & Inventory section of the product details page, along with other price values.
To set cost prices for product variants, use the Cost price field in the Variants tab of the product details page.
The product listing page only displays the parent cost price and provides facilities to quick-edit the value.
A store automatically generates regular profit reports based on the stock price and cost difference. The stats data is available on the Statistics page of your store's Admin area.
Check the Gross profit line to see your net earnings for different periods.