Heroku Postgres has always been priced on platform convenience with very high margins. It's been many years now so I don't remember the exact numbers, but I moved a few databases from Heroku to AWS and reduced my DB costs ~90% (magnitude ~900/mo --> ~100/mo) for roughly the same specs. They probably have a lot of margins to eat into before they need to adjust prices.
We're using the highest tier Postgres instance at my work for one of our legacy Heroku apps and it costs thousands over what we'd pay for the equivalent on AWS directly.
I'm assuming this means that they are not providing any sort of guarantee on the amount of RAM available and packing these instances as tightly as they can.
“Amazon Aurora Serverless is an on-demand, autoscaling configuration for Amazon Aurora. It automatically starts up, shuts down, and scales capacity up or down based on your application's needs.”
