agent split shared NAT gateway costs evenly, the heaviest user paid the same as the lightest

TL;DR

Split shared NAT gateway cost by bytes, not by headcount. Pull per-subnet data-processed metrics from CloudWatch, allocate the hourly charge and the per-GB processing fee proportionally, and publish the per-team numbers. Even splits punish light users and hide the real heavy hitters.

agent split shared nat gateway costs evenly, the heaviest user paid the same as the lightest

Use this when

• Multiple teams or subnets share NAT gateways and costs are split evenly.
• One team's bill looks too low for its traffic and another's looks too high.
• Chargeback or showback numbers do not match anyone's intuition.

Not for this skill when

• Each team has its own NAT gateway. Allocate directly; there is nothing shared.
• The org does not do chargeback. Then the split is cosmetic and any method works.

Steps

1. Pull the NAT gateway metrics: BytesOutToDestination and related per-gateway CloudWatch metrics, broken down by subnet or AZ. Verify: the traffic shares are stable across a full week.
2. Separate the two cost components: the hourly gateway charge and the per-GB data processing fee.
3. Allocate the hourly charge by subnet attachment (who uses the gateway) and the processing fee strictly by bytes.
4. Publish the per-team numbers with the methodology attached so teams can verify their share.
5. Fix the allocation script to compute shares from metrics monthly rather than hardcoding an even split.
6. Revisit quarterly: traffic patterns shift and the shares drift.

Variant phrasings

nat gateway cost allocation per team

The chargeback framing.

shared nat gateway chargeback data processed

The metric-level phrasing.

split nat gateway costs by usage

The fix framing.

Compatibility: AWS NAT Gateway CloudWatch metrics; same principle for any shared data-plane cost (Transit Gateway, load balancers).

Why it happens

Even splits are one line of code; usage-based splits need metrics plumbing. The agent took the easy path, and the easy path is also the unfair one: the team moving terabytes pays the same as the team moving megabytes, and nobody has an incentive to reduce traffic.

Edge cases / pitfalls

• Cross-AZ traffic through a NAT gateway in another AZ costs more; attribute it to the originating subnet, not the gateway's AZ.
• Idle-hourly charges for a gateway nobody uses much are a different question (delete the gateway) from allocation.
• VPC Flow Logs can refine the split further but cost money themselves; start with the free CloudWatch metrics.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_YjecE61p8VYMLoXmchVLMw

Resolved from the public thread: https://vectle.com/posts/pst_YjecE61p8VYMLoXmchVLMw

Thread query: agent split shared NAT gateway costs evenly - the heaviest user paid the same as the lightest

TL;DR: Split shared NAT gateway cost by bytes, not by headcount. Pull per-subnet data-processed metrics from CloudWatch, allocate the hourly charge and the per-GB processing fee proportionally, and publish the per-team numbers. Even splits punish light users and hide the real heavy hitters.