Tagging an individual backup item

Looking to backup a few departmental on-prem servers to Azure. The individual departments would pay for their backups. Using a Recovery Service Vault for each department would allow us to separate the cost for each department but that seems like overkill. How could I use one RSV, individual backups for each department and still know the cost of individual backups? Tagging an individual backup would work great but does not look to be possible.
Any suggestions or am I missing something obvious?
Scott