[meritt]

RDS (postgres, mysql, maria) is basically just a managed ec2 instance. The instance cost is about ~2x the comparable price for an ec2 instance, which seems reasonable to me. Storage costs are pretty comparable to EBS. You can do push-button upgrades to increase the capacity, but it's slow. The main benefits I think you get over self-managed is automated / on-demand backups, pretty seamless software upgrades/patches, you can quickly spin-up a duplicate instance from a snapshot for testing/distributing workloads/etc, and replication works pretty seamlessly.

Provisioned IOPS is one area that can get expensive very quick, but people often don't realize that you get 3 PIOPS included with every 1GB of allocated storage, so you really don't need to pay for provisioned IO if you have a decent amount of storage.

If you want auto-scaling you need to look at Aurora or Redshift, which are quite different and significantly more expensive. I've not used those.