TerraformPilot

DevOps

Terraform Cost Estimation: Infracost, FinOps, and Budget Control

Estimate infrastructure costs before terraform apply with Infracost. See cost diffs in pull requests, set budget policies

LLuca Berton2 min read

The Problem

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You run terraform apply and get a $500/month surprise on next month's AWS bill. Nobody reviewed the cost before deploying.

The Solution: Infracost

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Infracost shows you the cost of your Terraform changes before you apply them:

$ infracost breakdown --path .
 
 Name                                    Monthly Cost
 
 aws_instance.web
 ├─ Instance usage (t3.large, on-demand)       $60.74
 ├─ root_block_device (gp3, 50 GB)              $4.00
 └─ CPU credits                                  $0.00
 
 aws_db_instance.main
 ├─ Database instance (db.r6g.large)           $131.40
 └─ Storage (gp3, 100 GB)                       $11.50
 
 aws_nat_gateway.main
 ├─ NAT gateway                                 $32.85
 └─ Data processed (100 GB)                      $4.50
 
 OVERALL TOTAL                                 $244.99

Install Infracost

# 
# macOS
brew install infracost
 
# Linux
curl -fsSL https://raw.githubusercontent.com/infracost/infracost/master/scripts/install.sh | sh
 
# Docker
docker run -it --rm infracost/infracost breakdown --path /code

Get a free API key:

infracost auth login
# Opens browser → sign up → key saved to ~/.config/infracost/credentials.yml

Basic Usage

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Cost Breakdown

# 
# Full cost breakdown
infracost breakdown --path .
 
# JSON output
infracost breakdown --path . --format json
 
# Specific Terraform plan
terraform plan -out=plan.tfplan
infracost breakdown --path plan.tfplan

Cost Diff (Most Useful)

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See cost impact of your changes:

# Compare current branch to main
infracost diff --path . --compare-to main
 Monthly cost will increase by $156.00
 
 + aws_instance.api
   +$60.74 (new resource)
 
 ~ aws_db_instance.main
   +$95.26 (db.r6g.large → db.r6g.xlarge)
 
 Monthly cost: $244.99 → $400.99

CI/CD Integration

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GitLab CI

# 
infracost:
  stage: validate
  image:
    name: infracost/infracost:ci-0.10
    entrypoint: [""]
  script:
    - infracost breakdown --path . --format json --out-file infracost.json
    - infracost output --path infracost.json --format gitlab-comment --out-file comment.md
    - |
      # Post comment to MR
      curl -X POST \
        -H "PRIVATE-TOKEN: ${GITLAB_TOKEN}" \
        -H "Content-Type: application/json" \
        -d "{\"body\": \"$(cat comment.md | jq -Rs .)\"}" \
        "${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/merge_requests/${CI_MERGE_REQUEST_IID}/notes"
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"

GitHub Actions

# 
- name: Infracost
  uses: infracost/actions/setup@v3
  with:
    api-key: ${{ secrets.INFRACOST_API_KEY }}
 
- name: Generate cost estimate
  run: |
    infracost breakdown --path . --format json --out-file /tmp/infracost.json
    infracost comment github --path /tmp/infracost.json \
      --repo $GITHUB_REPOSITORY \
      --pull-request ${{ github.event.pull_request.number }} \
      --github-token ${{ github.token }}

This posts a cost table directly in your pull request.

Cost Policies

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Block Expensive Changes

# 
# infracost.yml
version: 0.1
projects:
  - path: .
 
policies:
  - name: "Monthly cost limit"
    description: "Total monthly cost must be under $1000"
    resource_type: "*"
    condition:
      monthly_cost: "<= 1000"
infracost breakdown --path . --format json | infracost policy check --policy-file infracost.yml

Budget Alerts by Tag

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Tag resources with cost centers:

resource "aws_instance" "web" {
  instance_type = "t3.large"
 
  tags = {
    CostCenter = "engineering"
    Project    = "api"
  }
}

Then track costs by tag in your FinOps dashboard.

Cost Optimization Patterns

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Right-Size Instances

# 
# Check: are you over-provisioned?
variable "instance_type" {
  type    = string
  default = "t3.micro"  # Start small, scale up based on metrics
 
  validation {
    condition     = contains(["t3.micro", "t3.small", "t3.medium", "t3.large"], var.instance_type)
    error_message = "Use approved instance types only."
  }
}

Use Spot/Preemptible for Non-Critical

# 
resource "aws_spot_instance_request" "worker" {
  ami                    = var.ami_id
  instance_type          = "c5.xlarge"
  spot_price             = "0.08"  # 60-90% savings
  wait_for_fulfillment   = true
  instance_interruption_behavior = "stop"
}

Schedule Non-Production

# 
# Tag dev/staging for auto-shutdown
resource "aws_instance" "dev" {
  tags = {
    AutoStop  = "true"
    Schedule  = "office-hours"  # Used by AWS Instance Scheduler
  }
}

Reserved Instances for Steady State

#
Pricingt3.large monthlySavings
On-Demand$60.74—
1-Year Reserved (No Upfront)$38.6936%
1-Year Reserved (All Upfront)$35.0442%
3-Year Reserved (All Upfront)$22.3463%

gp3 Over gp2

# 
# gp3 is 20% cheaper with better baseline performance
root_block_device {
  volume_type = "gp3"  # Not "gp2"
  volume_size = 50
}

Common Resource Costs (us-east-1)

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ResourceTypical Monthly Cost
t3.micro$7.59
t3.large$60.74
NAT Gateway$32.85 + data
ALB$16.43 + LCU
RDS db.t3.medium (Postgres)$49.06
RDS db.r6g.large (Postgres)$131.40
S3 (100 GB)$2.30
EBS gp3 (100 GB)$8.00
ElastiCache cache.t3.medium$46.72
Lambda (1M requests)$0.20

Biggest surprises: NAT Gateways (data transfer costs), ALBs, cross-AZ traffic.

Hands-On Courses

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Learn by doing with interactive courses on CopyPasteLearn:

Conclusion

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Install Infracost, add it to your CI pipeline, and see cost diffs on every pull request. Right-size instances, use gp3 over gp2, schedule non-production shutdowns, and watch out for NAT Gateway data transfer costs. The goal: no surprise AWS bills after terraform apply.

#Terraform#FinOps#Cost Optimization#Infracost#DevOps

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