This is for my personal lab. I am at my wits end trying to hunt down this cost. I do not have an EIP or public IP address or a NAT gateway. I do have ipv6 egress only gateway and EIPs for SQS, ECR and S3. I am mostly using my AWS for batch processing where I upload data to S3 and triggers a job by placing a message in a SQS queue. My VPC has subnets in different AZ.
I had a $5 a month VPS that cost $300 a month to run because it didn't have enough RAM and was swap-happy and running up incredible I/O costs against EBS. The $10 a month VPS was a lot cheaper.<p>In your case a Lambda could be cheaper still.