There are lots of alternatives for database solutions with Kubernetes, in Hopsworks we use RonDB which is highly available and has a MySQL interface but also supports a REST API and even a prototype Redis service.
I guess I am saying it's a good value more than its raw price being less than other offerings.
The blue/green deploys, auto snapshotting, failover, upgrades, few-click resizing, etc justify the price vs running database servers (for me). I spend a lot of time looking for money in aws and never consider replacing RDS with anything else.