Cost Estimation

A Terraform plan tells you what will change but not what it will cost. A one-line edit — bumping an instance type, adding a NAT gateway per availability zone — can quietly add thousands of dollars a month, and the plan shows it as a single innocuous ~ or +. Infracost is the de facto tool for putting a dollar figure on a plan, typically as a pull-request comment showing the monthly cost delta alongside the diff.

The whole idea is to shift cost awareness left — into the review, where catching an oversized resource costs a comment, instead of onto the invoice, where catching it costs a month of overspend and an awkward conversation. Cost becomes one more thing a reviewer sees next to the destroy and replace lines, not a surprise at the end of the billing cycle.

The Problem

Plans speak in resources, not money. A change from db.t3.medium to db.m5.large reads as a one-line update; a forgotten count = 3 on a NAT gateway reads as three creates. Neither line says "this is roughly an extra 1,200 dollars a month." The cost is real and predictable, but it is invisible in the plan, so a reviewer focused on correctness approves it without ever seeing the price tag.

How Infracost Works

Infracost reads a Terraform plan — usually the JSON output of plan — identifies the priced resources in it, looks each up against the cloud provider's published rate cards, and reports a monthly estimate plus the diff against the prior state. It runs as a CLI step in the pipeline, so the cost figure is computed from the same plan a reviewer is about to approve, not a hand-built guess.

# produce a machine-readable plan
terraform plan -out=tfplan
terraform show -json tfplan > plan.json

# price it and post the monthly delta to the PR
infracost diff --path plan.json \
  --format json --out-file infracost.json
infracost comment github --path infracost.json \
  --repo acme/infra --pull-request $PR_NUMBER

The pipeline turns the plan into JSON, hands it to infracost diff to price against the rate cards, and posts the result as a PR comment. The reviewer sees a line like "Monthly cost will increase by 1,180 dollars (+34%)" sitting next to the code diff — the financial impact made as visible as the resource changes.

PR Integration

The value lands when the cost delta is a comment on the pull request, beside the diff, where it is read during review rather than discovered later. A reviewer approving a change now sees both what changes and what it costs in one place. That co-location is the point: cost stops being a separate quarterly finance exercise and becomes part of the same review that catches a bad replacement.

Policies and Thresholds

A cost number that nobody acts on is noise. Set a threshold — flag or fail the PR when the monthly increase exceeds, say, 500 dollars or 20 percent — so a large jump demands an explicit sign-off rather than slipping through. Without a threshold, the estimate becomes wallpaper everyone scrolls past; with one, a meaningful increase forces a conscious decision before merge.

Limits of Estimation

Infracost prices what it can compute from the plan: instance hours, provisioned storage, gateway baselines. It cannot derive usage-based costs — data transfer, request volume, per-invocation Lambda charges — from the plan alone, because those depend on traffic the plan does not describe; Infracost estimates them only from a separately supplied usage file or default assumptions. Treat the estimate as a directional guide, not a guarantee. It will reliably catch an oversized instance or a multi-AZ NAT; it will not tell you exactly what your data-transfer bill will be next month.