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.
I am in Cost Explorer. The Service that is costing $34 is VPC. I am not able to drill down into this service. I did deploy a IPV6 egress which is serverless. It is is supposed to be free or charge me for data egress only. But this VPC as been idle since I deployed it.
I have also dumped my cloudformation stack template. I am 100% convinced it is the IPV6 egress , which is supposed to be serverless and hence a surprise as I did not do much with this VPC in December
As per this document,
An egress-only internet gateway is for use with IPv6 traffic only. To enable outbound-only internet communication over IPv4, use a NAT gateway instead. For more information, see NAT gateways.
Pricing
There is no charge for an egress-only internet gateway, but there are data transfer charges for EC2 instances that use internet gateways. For more information, see Amazon EC2 On-Demand Pricing.
I did cloudformation stack deployments in December to my account. I am pretty sure that it is the ipv6 egress , which is supposed to be serverless and hence cheaper that NAT, that is responsible.
In your case a Lambda could be cheaper still.