The Authorize.Net Customer Information Manager (CIM) allows you to store customer credit card information so future payments can be semi-automated. You'll then create your own recurring billing service (could be a cron job) that determines the amount to be charged (since it fluctuates monthly) and then calls the appropriate customer profile and charges them that amount. No human interaction required.
Otherwise no payment gateway offers any service that allows for automated flexible payments since it has no way to know what the payments will be ahead of time.
