How do I rotate old log files to long-term backup?

I have logrotate set up to archive logs for 30 days; how do I set up my cron job to rotate them automatically from /var/log/net to the long-term storage I have mounted on /mnt/backup?

I do not need to mangle the name when I archive them.

EDIT:

Example of file naming… wireless.log-20120916.gz… there is no fixed log name to key from (which is the assumption in Nikolaidis Fotis’ first answer)

You could create a second logrotate configuration and use as post script something like

#!/bin/bash
LogDate=$(date +"%s")
mv /var/log/messages.1 /data/logs/local_backup/var/log/messages/messages.$LogDate
gzip /data/logs/messages.$LogDate
exit

like here http://www.ashishnepal.com/logrotate-and-move-to-backup-directory/

EDIT

New approach …

/bin/find $path -mtime 29 -exec cp -p {} /newPath/ \;

you can either execute it from cron job or post process in logrotate

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