Hi,
AFAIK, the data lake storage cost is seperate to the original Data Lake Architecture solution and it is much lower than compared to Data warehouse solution.
Also you can check here - https://learn.microsoft.com/en-us/azure/storage/blobs/data-lake-storage-introduction
So any data stored via the solution you used will be charged accrodingly as per the usage such Hdinsight or Data Lake store - https://azure.microsoft.com/en-gb/pricing/details/storage/data-lake/
Hope this helps.
JS
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