I just got curious during handling the invoice of AWS. My gut feeling is that all inbound traffic are free (mostly), for example, you can see this in "Amazon EC2 On-Demand Pricing":
Data Transfer IN To Amazon EC2 From Internet
All data transfer in $0.00 per GB
But if you're dealing with inbound traffic across AZ in the same region:
Data transferred "in" to and "out" from Amazon EC2, Amazon RDS, Amazon Redshift, Amazon DynamoDB Accelerator (DAX), and Amazon ElastiCache instances, Elastic Network Interfaces or VPC Peering connections across Availability Zones in the same AWS Region is charged at $0.01/GB in each direction.
Yeah, inbound traffic across AZ is more expensive than the one from internet (which is free).
Also, inbound traffic to public IPv4 addresses and/or Elastic IP are not free:
IPv4: Data transferred “in” to and “out” from public or Elastic IPv4 address is charged at $0.01/GB in each direction.
IPv6: Data transferred “in” to and “out” from an IPv6 address in a different VPC is charged at $0.01/GB in each direction.
Anyway, this reminds me that I need to count US$0.02/GB instead of US$0.01/GB for such sort of traffic.