How to use Infracost with Atlantis for automatic cost estimation?

Question

Accepted Answer

**Infracost + Atlantis** — automatic cost estimation in PR.

**atlantis.yaml:**
```yaml
version: 3
projects:
  - name: infrastructure
    dir: infrastructure
    workspace: default
    autoplan:
      when_modified: ["*.tf", "*.tfvars", "atlantis.yaml"]
      enabled: true
    workflow: terraform-with-infracost

workflows:
  terraform-with-infracost:
    plan:
      steps:
        - init
        - run: infracost diff --path $ATLANTIS_PROJECT_DIR --format json --out infracost.json
        - run: infracost comment github --path infracost.json --repo $ATLANTIS_GIT_CLONE_URL --pull-request $ATLANTIS_PULL_NUM --behavior update
        - plan:
            extra_args: ["-lock=false"]
    apply:
      steps:
        - apply
```

**Infracost usage file (custom pricing):**
```yaml
version: 0.1
usage:
  aws_instance.web:
    operating_system: linux
    monthly_hrs: 730
  aws_s3_bucket.data:
    storage_gb: 100
    standard:
      monthly_tier_1_requests: 10000
      monthly_tier_2_requests: 1000
```

**PR comment example:**
```
🪙 Infracost estimate: monthly cost will increase by $12.41 (+8.5%)

| Product                | Current  | Change  | New      |
|------------------------|----------|---------|----------|
| aws_instance.web       | $8.47    | +$3.94  | $12.41   |
| aws_s3_bucket.data     | $0.00    | +$2.30  | $2.30    |
| TOTAL                  | $8.47    | +$6.24  | $14.71   |
```

**Pros:** Cost visibility in every PR, automatic comparison, guardrails (block expensive resources).

DevOps: How to use Infracost with Atlantis for automatic cost estimation?