There really is no easy way of determining the average CPU/memory utilization for a month. The firewall's audit will log a system health entry every minute. This entry provides the current CPU and memory usage, load averages, and other information as well. You could use this audit entry to determine the average CPU/memory utilization manually.
To extract these audit entries from a given audit.raw file, run the following command:
acat -ae 'area AUDIT_A_HMONINFO' /var/log/<audit.raw.filename>
By default, the firewall only retains 20 instances of the audit.raw file, so going as far back as a month is unlikely unless you modified the amount of copies to retain.