DU Meter supports calculation of monthly data transfer costs if your provider bills you per megabyte. This feature is configured in DU Meter Options. Here are some finer points that our customers had issues with:
Change of per-megabyte cost or reduced rate definition will apply to the future traffic only. It will not be applied retrospectively to old data. If your provider changes their rates, please update DU Meter configuration when the actual rate change becomes effective.
New free allowance with the amount you just entered will be created for the current month, in addition to whatever free allowance bytes were used previously.