@DannyF
Pricing information for Azure Files and File Sync can be found here. Sync uses file operations to sync your data to the cloud and other servers, and you will be billed for any operations that are generated in the sync process. So if the files is already cached locally you would not expect a charge. If the file needs to be pulled from the file share you will be charge for that transaction and data transfer.
Hope this helps. If you have further questions regarding Billing or pricing I recommend creating a support ticket with or Billing team. It is free and the best option for this case.
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