Illustrative example · not from a client engagement · simplified 12-month figures
S3 traffic from private subnets paying NAT gateway processing charges
- EvidenceThe billing lines and resource metadata it rests on
- 20,000 GB a month billed as NatGateway-Bytes in us-east-1. Private-subnet route tables send 0.0.0.0/0 to NAT gateways and have no S3 gateway endpoint.
- Dollar impactAnnualized, priced from your billing data where available
- 12,000 GB × $0.045/GB = $540 a month; × 12 = $6,480 a year (simplified). A gateway endpoint for S3 has no hourly or data-processing charge.
- AssumptionsEvery input the number depends on
- 60% of NAT-processed bytes are same-Region S3 traffic (to be measured). $0.045 per GB processed is the us-east-1 list price. Volume stays flat.
- ConfidenceHow sure, and what would change it
- Medium. Price and volume come straight from billing data; the S3 share is an estimate until your team confirms it from VPC Flow Logs, which the audit role cannot read. In a live report, a value resting on an unmeasured assumption like this may be stated as a range, and only the lower bound of a range counts toward the guarantee.
- EffortWhat it takes to capture
- Low. One gateway endpoint per affected VPC plus route-table associations: a few lines of Terraform or CloudFormation.
- Production riskWhat could break, and who should review it
- Low to moderate. Switching routes drops open TCP connections to S3. Bucket or IAM policies matching aws:SourceIp stop matching (use aws:VpcSourceIp). Same-Region S3 only.
- RecommendationThe specific change
- Create an S3 gateway endpoint in each affected VPC and associate the private route tables, one VPC first, outside peak hours.
- Suggested validation & rollbackGuidance for your team, which implements, validates and rolls back
- Your team can validate that NatGateway-Bytes drops by roughly the S3 share in Cost Explorer within a day or two, and that S3 clients reconnect cleanly. To roll back, your team disassociates the route tables or deletes the endpoint, and traffic returns to the NAT gateway.
Figures are computed from the stated assumptions, not measured in any client account. This is a simplified illustration (monthly figure × 12), not the guarantee calculation, which the engagement agreement defines using a baseline period, normally the most recent completed AWS billing month. Price: AWS Price List API, AmazonEC2 us-east-1, NatGateway-Bytes, effective 1 Sep 2026. Gateway-endpoint pricing and routing behavior: AWS PrivateLink documentation, “Gateway endpoints for Amazon S3”.
