To lower your MongoDB backup bill, you can use MongoDB Management Service (MMS) to back up your data with a pay-as-you-go model that combines snapshot storage and oplog processing fees. MMS offers a free tier to get started, allowing you to monitor your usage and estimate your monthly costs. You can further optimize your costs by excluding non-critical collections from backup, which will reduce the amount of data being backed up, or by setting a custom snapshot schedule and retention policy, allowing you to control when and how long snapshots are retained. By doing so, you can ensure that only mission-critical data is included in backups, reducing unnecessary storage costs.