Yes, the monthly payment has been suggested in the past, and Scott did allow it once for the memberships and the private messages, BUT the obvious happened. People signed up, checked their messages, then immediately cancelled. As this is all done manually, it was creating major headaches administratively.
The best option is really to sign up for a Gold membership and get the benefits of that as well, including the Challenge.
If the messages are driving you nuts, you can sort of tell by the email notification you received whether it may be worth checking into. Their are writing opportunity ones that will say that in the notification, and they are sent out to a broad number of members. The other type are most often from someone contacting you after reading something you wrote (most often by other members).