Turn on committed use discount sharing, and create a commitment for the combined usage

Sharing your committed use discounts across all your projects reduces the overhead of managing discounts on a per-project basis, and maximizes your savings by pooling all your discounts across your projects' resource usage. If you have multiple projects that share the same Cloud Billing account, you can enable committed use discount sharing so all of your projects within that Cloud Billing account share all of your committed use discount contracts. Your sustained use discounts are also pooled at the same time. That is, sustained use discounts are calculated using the total resources across these projects, rather than just the resources within a single project.

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Reference link- https://cloud.google.com/compute/docs/instances/signing-up-committed-use-discounts#sharing_committed_use_discounts_across_projects