cost-impact-summary

name	cost-impact-summary
description	Use when asked about cost impact of a change, "what does this cost", "cost estimate", or "/cost-check". Summarizes the AWS/GCP/cloud cost implications of an infra or code change.

cost-impact-summary

When to use

• "what does this change cost?" / "/cost-check"
• "cost impact of this PR"
• "estimate the infra cost"
• "is this expensive to run?"

Inputs

• Terraform plan output, PR diff, or a description of the infra change.
• Optionally: current AWS/GCP account, region, expected traffic/load.

Procedure

1. Identify billable resource changes from the plan/diff:

   • Compute: EC2, ECS tasks, Lambda, GKE nodes
   • Storage: S3, EBS, RDS, ElastiCache
   • Network: NAT Gateway, data transfer, load balancers, CloudFront
   • Other: managed services (RDS, MSK, OpenSearch, etc.)

2. For each changed resource, estimate:

   • Current cost (if it's a modification).
   • New cost (size, count, or tier change).
   • Delta (+ increase / - savings).

   Use rough public pricing. Flag when a precise estimate requires Infracost or AWS Pricing Calculator.

3. Highlight cost surprises:

   • NAT Gateway data processing charges (often overlooked).
   • Cross-AZ or cross-region data transfer.
   • Provisioned IOPS changes.
   • Reserved/Savings Plan coverage gaps.
   • Lambda duration vs. provisioned concurrency trade-offs.

4. Output:

   ## Cost Impact Summary
   
   | Resource | Change | Est. Monthly Delta |
   |---|---|---|
   | `aws_instance.app` | t3.medium → t3.large | +$29/mo |
   | `aws_nat_gateway` | new | +$32/mo + transfer |
   | `aws_rds_instance.main` | db.t3 → db.m5 | +$85/mo |
   
   **Estimated total delta:** +$146/mo
   
   ### Watch out for
   - NAT Gateway data transfer charges depend on traffic volume — estimate based on X GB/day.
   - `aws_rds_instance` upgrade requires a maintenance window.
   
   ### For precise numbers
   Run: `infracost breakdown --path .`
   Or: AWS Pricing Calculator for <resource>
   

Guardrails

• Estimates are rough — always caveat with "verify with Infracost or AWS Cost Explorer."
• If infracost is available in the environment, prefer running it over manual estimation.
• Do not approve or reject a change based on cost alone — surface the data, let the user decide.