Hi,
Here is what I can explain you on your questions. Yes, there is a charge associated with SFTP enabling. It's $0.300 per hour as per Azure Prizing Calculator
On your question on calculation of SFTP charges while the feature is turned off, here are the details: Reference: Pricing and billing
So, SFTP can be switched off with 1 click and the passive charges can be avoided
Hope this helps.
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