Here are ten ways to reduce hosting costs for MongoDB deployments:

1. Right-size your instances: Choose the appropriate instance size based on your workload requirements to avoid overprovisioning and paying for unused resources.

2. Use reserved instances: By committing to a longer-term usage of instances, you can take advantage of reserved instance pricing, which offers significant discounts compared to on-demand pricing.

3. Implement automatic scaling: Utilize auto-scaling features provided by cloud platforms to automatically adjust the number of instances based on the workload, ensuring you only pay for the resources you need.

4. Optimize indexes: Proper indexing can significantly improve query performance and reduce the need for excessive compute resources, thereby lowering costs.

5. Enable compression: MongoDB supports data compression, which can reduce storage costs and improve performance by reducing the amount of data transferred over the network.

6. Leverage spot instances: Some cloud providers offer spot instances, which are unused compute capacity available at a discounted price. If your workload is fault-tolerant, you can utilize spot instances to save costs.

7. Monitor and optimize queries: Regularly monitor slow queries and optimize them to reduce resource consumption. Efficient queries can help minimize the need for additional compute resources.

8. Use sharding for horizontal scaling: Sharding allows you to distribute data across multiple instances, enabling horizontal scaling. This can be more cost-effective than scaling vertically with larger instances.

9. Implement backup and retention policies: Regularly review and optimize your backup and data retention policies to ensure you are not storing unnecessary data, which can increase storage costs.

10. Consider serverless options: Some cloud providers offer serverless MongoDB solutions, such as AWS DocumentDB or Azure Cosmos DB, which can automatically scale based on demand and charge based on actual usage, potentially reducing costs for variable workloads.

Remember to regularly monitor your usage, analyze costs, and make adjustments based on your specific requirements and usage patterns to optimize your MongoDB hosting costs in the cloud.