We have multiple "customers" logging into our Splunk environment and we need to be able to track their license and storage consumption. There must be a way built into Splunk that can allow us to do this, we are in the process of setup, so the time is now 🙂
You could put each client in their own index (cust1_web, cust1_sales, etc) and then the license usage split by index would help you.
You could also put each of them in their own drive or filesystem structure (e:\cust1\splunkdb, e:\splunkdb\cust1, /cust1/splunkdb, etc.) to give you easy OS insight to their storage total consumption too. (I have a Powershell input that tracks storage usage of directories, so you can get that OS info right through Splunk, too.)