Cost Estimation

TerraCi can estimate the monthly cost impact of infrastructure changes by analyzing Terraform plans against AWS pricing data. Cost estimates are calculated per module and displayed alongside plan results in MR comments.

Basic Configuration

extensions:
  cost:
    providers:
      aws:
        enabled: true

This is the minimum needed to opt in. Pricing data is cached automatically via the diskblob backend under the project's service directory.

Configuration Options

providers.aws.enabled

Enable AWS cost estimation. The plugin uses EnabledExplicitly activation, so cost runs only when at least one provider's enabled flag is true.

extensions:
  cost:
    providers:
      aws:
        enabled: true

blob_cache

Pricing data is cached via a blob store backend (diskblob by default). Override the backend, namespace, or TTL when needed:

extensions:
  cost:
    blob_cache:
      backend: diskblob       # default; any registered BlobStoreProvider
      namespace: cost/pricing # default
      ttl: "24h"              # default

ttl accepts Go duration strings: "1h", "30m", "72h", etc. The diskblob backend stores objects under ~/.terraci/blobs/ by default — see the diskblob plugin config to override.

Removed fields

Earlier versions accepted top-level cost.cache_dir and cost.cache_ttl. Both are gone — cache_dir is rejected at validation, and cache_ttl has moved to blob_cache.ttl.

How It Works

1. After terraform plan completes, TerraCi reads the plan.json file from each module directory.
2. Resource changes are extracted and matched against registered AWS resource definitions.
3. Pricing is fetched from the AWS Bulk Pricing API and cached via the configured blob_cache backend.
4. Per-resource hourly and monthly costs are calculated for both the before and after states.
5. Results are aggregated into a per-module cost summary with before/after/diff values.

Supported AWS Resources

Category	Resources
EC2	Instances, EBS volumes, Elastic IPs, NAT Gateways
RDS	Instances, clusters, cluster instances
ELB	Application Load Balancers, Classic Load Balancers
ElastiCache	Clusters, replication groups, serverless caches
EKS	Clusters, node groups
Serverless	Lambda, DynamoDB, SQS, SNS
Storage	S3, CloudWatch alarms/log groups, KMS keys, Route 53 zones, Secrets Manager

Each resource type has a dedicated definition that maps Terraform resource attributes to the corresponding pricing dimensions.

MR/PR Integration

When cost estimation is enabled, cost estimates appear in the MR/PR comment table. Each module shows its monthly cost difference:

### 💰 Cost Estimation

| Module | Monthly Before | Monthly After | Diff |
|--------|---------------|---------------|------|
| platform/prod/eu-central-1/vpc | $120.50 | $185.30 | +$64.80 |
| platform/prod/eu-central-1/eks | $450.00 | $450.00 | $0.00 |
| **Total** | **$570.50** | **$635.30** | **+$64.80** |

Full Example

execution:

extensions:
  cost:
    blob_cache:
      backend: diskblob
      namespace: cost/pricing
      ttl: "24h"
    providers:
      aws:
        enabled: true

  # MR/PR comments are rendered by summary.
  summary:
    enabled: true

This configuration enables cost estimation with default caching, and displays the results in MR/PR comments alongside plan output.

CLI Command

Run cost estimation directly from the command line:

# Estimate all modules with plan.json
terraci cost

# Estimate a single module
terraci cost --module platform/prod/eu-central-1/rds

# JSON output
terraci cost --output json

# Verbose — shows per-resource breakdown and pricing cache info
terraci cost -v

The terraci cost command scans for plan.json files, fetches pricing data, and outputs per-module cost estimates. Pricing cache location and TTL expiration are shown in the output.

In JSON output, each resource now carries a status:

• exact for fully priced resources
• usage_estimated when TerraCi can derive a partial estimate from configured capacity
• usage_unknown when the resource still needs runtime usage data
• unsupported / failed with optional failure_kind and status_detail

Note: terraci cost requires extensions.cost.providers.aws.enabled: true in your .terraci.yaml.

In CI pipelines, cost estimation runs automatically as part of the terraci summary command (which posts MR/PR comments). Use terraci cost for local development and ad-hoc cost checks.

Examples

See examples/cost-estimation for a working example with VPC, EKS, and RDS modules.

See Also

• Summary Configuration — MR/PR comments with plan summaries and cost estimates
• Pipeline Generation Guide — end-to-end guide for generating CI pipelines