I've got two different copyright, one for my free plugins and applications (e.g. Guest Message, Donate Sidebar) that is 15 USD lifetime and covers only the free plugins/apps. I've got another copyright that is 45 USD lifetime and covers all my paid and free apps and plugins. (e.g. Donations, Forms, Videos, Guest Message, Donate Sidebar).
It's not a secret, I just don't sell it through the marketplace as it doesn't include the ability to track urls with copyright removal. I can manually setup an account if you can PM me the details you would like to use.