You can find the Bandwidth usage in cPanel servers using the script given below.
awk '{sum+=$10} END{printf("MB: %d\n", sum/(1024*1024))}' /usr/local/apache/domlogs/domainname
Eg:-
awk '{sum+=$10} END{printf("MB: %d\n", sum/(1024*1024))}' /usr/local/apache/domlogs/example.com
The output will be like given below.
MB: 6589
This means the bandwidth usage is around 6 GB.