If I'm understanding your question correctly, you might be able to change the AuthName on the fly if you put your authentication in .htaccess files in the directory (as opposed to the global httpd.conf file). However, you then run the risk of someone who has paid logging into that directory at the wrong time and also getting the "not paid" message.

Off the top of my head, I would say your best bet is to try to alert the users of non-payment, would be to redirect them to an alternate "error" page when you decline their authorization.