To add your product costs, you can click the button in the top left corner of a product card and then you'll see a button which is edit product button. You can click that and then fill the cost box with your product cost and save it, that's all. This is how to add a cost to an existing product.
Apart from this, you can also add your product costs while adding your products to the system. When you click add single product button, you'll see the cost box, just write your cost there and save it, that's all! The video below shows the details of these processes in an example account.