We’re now previewing the addition of PostgreSQL compatibility to Amazon Aurora. This edition has the benefits customers have come to expect from Aurora, including high durability, high availability, and the ability to quickly deploy low latency read replicas while supporting the full SQL dialect and functionality of PostgreSQL 9.6.
https://aws.amazon.com/blogs/big-data/join-amazon-redshift-a...
db.r3.8xlarge: RDS Aurora $4.640/h, RDS MySQL $3.780/h, RDS PostgreSQL $3.980/h
> db.r3.8xlarge: RDS Aurora $4.640/h, RDS MySQL $3.780/h, RDS PostgreSQL $3.980/h
Based on your numbers the difference in pricing between Aurora and Postgres ($4.64 - $3.98) x 24 * 30 = $475/mo. To the company using a db.r3.8xlarge, which has 32-cores and 244 GB of RAM, that's not even a rounding error.
That alone is worth the difference imo.