You can now get granular visibility into Kubernetes costs in Cost analysis within Azure portal. You can view the aggregated costs for all your clusters in a subscription and costs of all the namespaces for your clusters. All you need to do is to enable the AKS cost analysis add-on for your clusters. The AKS cost analysis add-on is built on top of OpenCost, an open-source Cloud Native Computing Foundation Sandbox project for usage data collection, which gets reconciled with your Azure billing data. Please refer to the articles below for installation of the add-on and accessing the views.
https://learn.microsoft.com/en-us/azure/aks/cost-analysis
https://learn.microsoft.com/en-us/azure/cost-management-billing/costs/view-kubernetes-costs