“The problem that I want to highlight today is that if I spin up MySQL myself on EC2 instances, I’ll have to pay 2¢ per GB that I replicate between AZs, whereas I will pay nothing if I use RDS.”
AWS charges 2 cents per GB to replicate data between availability zones, unless the service doing it has Amazon on the label. Self-hosted MongoDB pays it, Elastic pays it, Confluent pays it. DocumentDB, managed OpenSearch and RDS don’t. AWS tells you to run in multiple zones for reliability and then bills every competitor for following the advice.