Hello @John Wong ,
If I understand correctly, you are trying to automate the collection of cost breakdown by sub/resource group/ resources for over 100 subscription. If yes, then sounds like you need a scalable solution or script. I suggest exploring Cost management REST API or PowerShell module. You may need Enterprise level Agreement to consume the REST API operations but this equally possible with the PS module.
Let me know if this is what you are looking for. Happy to assist further.