If you establish a private link connectivity between your Azure subscription and the customer's Azure subscription, and then use that private link to connect to the customer's on-premise database, there should not be any egress costs associated with the data flow.
Private Link allows you to securely access services over a private network connection, bypassing the public internet. In this scenario, the data flow from the customer's on-premise database to your Azure subscription will be routed through the Express Route connection and the private link connectivity established between the two subscriptions.
Since the data flow occurs within the Azure network infrastructure and doesn't traverse the public internet, there should not be any egress costs involved. However, it's important to note that you may incur data transfer costs related to the Express Route connection, as this connection utilizes dedicated bandwidth and has associated pricing based on factors such as the bandwidth capacity and data transfer volume.
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