@Anonymous Thank you for posting this question here.
Based on the information available in question, I understand that you are using Legacy/classic Application Insights resource (and not the workspace based).
For this resource, the data ingestion pricing per GB from one Application Insights instance can be different from the data ingestion pricing of another Application Insights instance as a result of the pricing tier that each instance is in, ie., Per Node OR Per GB. Please see this link for more details - Legacy pricing tiers
In case they both are using the same pricing tier, I would suggest reaching out to Azure Billing Support which is available for all Azure customers. They would be able to help understand the billing and also rectify if needed.
Hope this helps. Please let us know if you have any questions.
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