Amazon Relational Database Service (Amazon RDS) is a collection of managed services that makes it simple to set up, operate, and scale databases in the cloud. Amazon RDS frees you to innovate and build new applications without worrying about the management of underlying database resources.
Amazon RDS is free to try and you pay only for what you use with no minimum fees. Pricing is based upon several factors including instance type, number of instances, database engine and storage options.
The first factor is On Demand vs Reserved Instance pricing. A Reserved Instance is a resource that is provisioned and leased for a period of 1 or 3 years, with significant cost savings.
For example, an On-Demand db4.m4.large PostgreSQL instance costs $0.364/hour. To run 3 instances in a cluster the total cost is $1.092/hour or $797.16/month using the On Demand pricing model. A 1-year Reserved Instance lease with no upfront payment reduces that cost to $549.60/month
There are separate calculations based on storage options for your RDS instances — General Storage (GS) and Provisioned IOPS you pay for the amount of storage provisioned per month at a rate of $0.23/GB/month. Continuing with the above example, 100GB of General Storage would ultimately cost $69/month ($.23/GB/month x 3 instances)
With Provisioned IOPS you pay for storage ($0.25/GB/month) and you pay for the number of IOPS you have provisioned ($0.20/Provisioned IOP). For the same scenario above the total storage cost comes to $675/month.