On the environments we take over, savings almost always come from the same line items, in this order.
Non-production environments left running. Development, testing and staging often run continuously when they are only used during business hours. Automatic shutdown typically saves 50 to 80% on those environments.
Sizing. Instances calibrated on a load estimated before go-live and never revisited.
Forgotten storage. Detached volumes, backup snapshots with no expiry policy, objects with no lifecycle rule.
Data transfer. Rarely instrumented, and often the most surprising line once measured.
FinOps is included in our managed services contracts: this tracking is not a separate engagement to trigger once the bill has already drifted.