I have to search for it (again) but there was someone a couple of years ago who made a really nifty script which would do all the collecting and processing and send out info via mail per period you could specify yourself.
Used it for SXT LTE6 setup in South of France to track data usage for vacation house tenants (limited monthly data volume, place is sold now).
Ah, found it !
http://forum.mikrotik.com/t/per-ip-wan-traffic-statistics/157975/1