What if users were given a salary based on activity? It would go up in rank (X$/hour for user, 1.5X$/hour for VIP, 2X$/hour for Super, 2.5X$/hour for Goldy, 5X$/hour for Moderator, infinite $ for admin regardless), or something like that. That would encourage activity and buying higher ranks. The reason I give mods a much higher salary is because they actually have a job to do and so that they can use $ as a reward for events. Admins could also give $ rewards for events, but it wouldn't be a major source of inflation due to the rarity of these events.
Then, of course, to avoid inflation, we have the server shop, a nice money drain.