I don't believe DA (yet) saves previous month's statistics.
You could, though; it would be easy enough to create a script that would read the contents of the bandwidth.tally files for all the users, and send them to you in an email.
The bandwidth tally files are at:
/usr/local/directadmin/data/users/*/bandwidth.tally
Or you might want to keep them on the server; in which case you could create your own logrotate script (the system's logrotate wouldn't work, because it wouldn't do it at the right time).
If I were going to do that I'd probably run them at 23:55, though, to give the server enough time to finish even if it ran into some kind of delay issue.
And while you were at it, you could create a script to give you usage tallies as well.
Note that writing such scripts is NOT on my todo list, since I believe DA has said they'll eventually offer the functionality.
Jeff