The problem is rarely cost, it's operational overhead.

Using SQS for a queue rather than my already-existing Postgres means that I have to:

- Write a whole bunch of IaC, figuring out the correct access policies - Set up monitoring: figure out how to monitor, write some more IaC - Worry about access control: I just increased the attack surface of my application - Wire it up in my application so that I can connect to SQS - Understand how SQS works, how to use its API

It's often worth it, but adding an additional moving piece into your infra is always a lot of added cognitive load.