There are the same total rewards every day which are distributed equally to all the verified emails. If the number of LISTNERDS for individual mails is increasing, it is because the number of verified mails is decreasing.
It's not clear whether there are less mails overall, or less mails getting verified, or what is causing this. One of the factors, as you say, will be the price of LISTNERDS.
I'm proposing a community meeting of LISTNERD holders in this post. It would be great to have your feedback.