It's no. 3 from Marek's list:
3. Go to Reports section, Tape Written and search for tapes written by Media Server. There is Kilobytes field.
Tape written report uses bpimagelist.
Try: bpimagelist -l -media -d <start-date> -e <end-date>
Field 5 is the kilobytes written.
Obviously only valid images will be reported on. If daily backups has a 2-week retention period, running the report for the last month will not contain info for the dailies that had already expired.