Have you checked out Cost Anomaly Detection[1]? It builds an ML model to alert on anomalous usage and resulting changes in billing.
[1] https://aws.amazon.com/aws-cost-management/aws-cost-anomaly-...
[1] https://aws.amazon.com/aws-cost-management/aws-cost-anomaly-...
These products have their place, but they don't make sense until you reach a certain size.