Infracost scans for Terraform code changes and checks over 3 million prices to create a simple, understandable cost estimate before any resources are launched.

Infracost Community

I believe the stand out diff between GCP billing(forecasted) and infracost(estimated) is reflection of changes in costs as soon as current real world infrastructure provisioned by Terraform is either modified or added.
GCP Billing updates the billing account after some time and ‘never’ before the resources are provisioned, to accomplish that we are recommended to use Pricing calculator separately.

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