You can export the products where the Cost field is blank. After you have input the Cost field you can edit and upload back into StoreAutomator.
1- Select the related products and go to Export>No Cost.
2- Download the file from the Notifications on the top right.
These are the fields that you can edit.
IMPORTANT NOTE: If you want to delete a specific field data, you must leave the cells as empty. This action will delete the data which you have already in SA, when you upload the file back into SA.
3- After the changes, upload the file back into SA.