The RDS FAQ reveals this: Q: Why is the pricing for each RDS database engine different? The pricing for each database engine of RDS varies because our costs a

The RDS FAQ reveals this: Q: Why is the pricing for each RDS database engine different? The pricing for each database engine of RDS varies because our costs are different for each. These costs include many operational components in addition to software licensing. We will continue to work hard to reduce costs and pass on those savings to our customers. My guess is that Postgresql required them to write much more internal tooling to "get right", and the costs reflect that. See also: https://aws.amazon.com/rds/faqs/ https://aws.amazon.com/rds/faqs/