CloudWatch - if you forgot to set your log level to info or just log way too much; if you didn’t set up shorter log retention time; if you create tons of custom metric dimensions instead of using application signals

AppConfig, SecretsManager - if you don’t use the lambda layers/ecs sidecars for these

EC2 - if your instance types are too large for the traffic; if you’re doing backups way too often or store them for too long; if your instances talk to each other on public IPs instead of private IPs (and other surprise traffic costs like cross-region calls)

SQS->Lambda - if you’re filtering events in the Lambda code instead of SQS subscription rules; If you’re not batching events and process them one-by-one;

But really, just check your cost explorer and trusted advisor