Simplest way to do this would be to add a simple queue to your metered DSL lines and keep track of the data recorded by this, noting this value will reset if the router is repowered.
Alternatively you might be able to respurpose a script like this: http://www.mikrotik-routeros.com/?p=24 to record and report back to you when your links reach certain limits.
This script also records the data in the comment field so negates majority of any lost data due to a router reboot.