Need help in understanding RDS Cluster Costs

I understand your confusion about RDS cluster costs versus instance costs. Let me clarify this for you.

First, it's important to note that RDS clusters and instances are priced differently, and the cost structure depends on the configuration you choose.

For a single RDS instance and a cluster with one visible instance on the console, the costs would generally be similar if they have the same instance type (like db.t3.xlarge in your example). However, clusters typically have some additional overhead costs associated with the cluster management.

When it comes to Multi-AZ deployments, there's a significant difference. A single RDS instance with Multi-AZ will cost approximately twice as much as a single instance without Multi-AZ, because you're essentially running two instances (primary and standby). A cluster with one visible instance and Multi-AZ would also cost more than a single instance, but the pricing structure might differ slightly due to how clusters manage high availability.

For clusters with multiple visible instances, each instance is billed separately. So a cluster with 2 visible instances would cost roughly twice as much as a cluster with 1 instance of the same type. If it's Multi-AZ, each of those instances would have a standby, further increasing the cost.

A cluster with 3 visible instances would, similarly, cost about three times as much as a cluster with 1 instance. In a Multi-AZ setup, each of these instances would have a standby, effectively doubling the cost again.

When you have Multi-AZ across more than one zone, the pricing remains consistent with the number of instances and their Multi-AZ status. The additional zones don't typically incur extra instance costs, but there might be additional data transfer costs to consider.

Remember that these costs are for compute resources. You'll also need to factor in storage costs, which depend on the type and amount of storage you provision, as well as potential costs for features like Performance Insights or Enhanced Monitoring if you choose to use them.

To get an accurate estimate of costs for your specific use case, it's recommended to use the AWS Pricing Calculator, which can provide a detailed breakdown based on your exact configuration.
Sources
On-Demand DB instances for Amazon RDS - Amazon Relational Database Service
Reviewing RDS cost break down | AWS re:Post
Amazon RDS for PostgreSQL Pricing – Amazon Web Services

Hi, DB Clusters are made with Amazon Aurora. Multi-AZ deployments are created with RDS. With Multi-AZ you can have one or two standby DB instances.

Multi-AZ with one standby instance is called Multi-AZ DB instance deployment. Multi-AZ with two standby instances is called Multi-AZ DB cluster deployment.

Additionally, not all DB instance types are available for Mutli-AZ. See Instance Availability for Multi-AZ Clusters for more information.

You referenced the RDS for PostgreSQL pricing page so I will use that as a reference. The region you launch the RDS resources in will impact the cost, so I will use US-East(N. Virginia) as an example. I also changed the instance type to db.m6gd.large since this instance type is available for most use cases. Aurora does not have this type, so I will use db.t3.large which has the same vCPU and Memory as a db.m6gd.large instance. In US-East(N. Virginia) as of Feb 2025:

Aurora Pricing Page for more details on the Aurora cost. Using the pricing chart above, the lowest cost would be a single DB instance. The cost will increase as you use different RDS/Aurora features. Creating a cluster around a DB Instance with Multi-AZ will increase cost depending on how many other instances there are.

There are other costs associated that can contribute to your final bill like RDS Storage costs, more details on this page DB instance billing for Amazon RDS. Lastly, You can use the AWS Pricing Calculator to make estimation of your total RDS cost.