When you charge away from home, whether it is at a public charger or at a friend's house, we often have no information about how much the session cost. In this case, we leave the cost of the session empty.
We are working on an improvement, so that in the future you will be able to edit the cost of the session directly in the app.
In the meantime, you can edit the CSV file, and update it with the cost indicated on the receipt you received from your charge point operator.