[0] https://dev.to/aws-heroes/100k-write-iops-in-aurora-t3medium...
I believe the article is talking about I/O aggregate operations vs I/O average per second. I'm talking strictly about the "average per second" variety. The former is really only relevant for billing in the standard billing mode.
Actually a big motivator for the migration was batch writes (we generate tables in Snowflake, export to S3, then import from S3 using the AWS RDS extension) and Aurora (with ability to handle big spikes) helped us a lot. We'd see application performance (query latency reported by APM) increase a decent amount during these bulk imports and it was much less impactful with Aurora.
iirc it was something like 4-5ms to 10-12ms query latency for some common queries regularly and during import respectively with RDS PG and more like 6-7ms during import on Aurora (mainly because we were exhausting IO during imports before)
If you have a PG cluster with 1 writer, 2 readers, 10Ti of storage and 16k provision IOPs (io1/2 has better latency than gp3), you pay for 30Ti and 48k PIOPS without redundancy or 60Ti and 96k PIOPS with multi-AZ.
The same Aurora setup you pay for 10Ti and get multi-AZ for free (assuming the same cluster setup and that you've stuck the instances in different AZs).
I don't want to figure the exact numbers but iirc if you have enough storage--especially io1/2--you can end up saving money and getting better performance. For smaller amounts of storage, the numbers don't necessarily work out.
There's also 2 IO billing modes to be aware of. There's the default pay-per-IO which is really only helpful for extreme spikes and generally low IO usage. The other mode is "provisioned" or "storage optimized" or something where you pay a flat 30% of the instance cost (in addition to the instance cost) for unlimited IO--you can get a lot more IO and end up cheaper in this mode if you had an IO heavy workload before
I'd also say Serverless is almost never worth it. Iirc provisioning instances was ~17% of the cost of serverless. Serverless only works out if you have ~ <4 hours of heavy usage followed by almost all idle. You can add instances fairly quickly and failover for minimal downtime (of course barring running into the bug the article describes...) to handle workload spikes using fixed instance sizes without serverless