The free sources are marked by Microsoft when ingested as IsBillable==false, you can't ingest a billable resource and make it free.
Billable data will be categorized as IsBillable==true. You do get the ingested data in the Workspace retained, for up to 90days as free (only when you have Sentinel associated to the Log Analytics workspace, you can change the retention setting from 30 --> 90days).
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