Hi,
Actual cost cannot be calculated exactly. If you enable AllMetrics they are mainly the same size every hour. Same can be said for Azure Policy Evaluation details. That will depend on the number of Key Vault specific Azure policies you have enabled. Most cost will come from Audit logs. Those solely depends on the number of operations on the secrets/keys. etc. For example even if you have 1000 secrets if nobody is accessing the secret you will not get much logs. If you have even 10 secrets but those are accessed constantly those 10 secrets will generate higher volume of logs compared to those 1000.
My suggestion is to enable the logs for one day and calculate the size of the logs so you can calculate the cost. That day of course should be normal working day instead of enabling them on Saturday or Sunday where usually the activity for most organization is not high. Usage can be calculated for logs. Of course there could be some periods where you have higher or lower cost but it should be sufficient for having some rough estimation.
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