Data Transfer Costs
Charges incurred when data moves out of AWS, between AWS regions, or between certain services: one of the least visible and most commonly underestimated line items on an AWS bill.
Definition
Data Transfer Costs apply when data leaves AWS to the public internet, moves between AWS regions, or crosses certain service boundaries within the same region. Inbound transfer is typically free; outbound and cross-region transfer are not, and the pricing is usually tiered and easy to miss when architecting a system, since it rarely shows up until the bill arrives. Architectures that route traffic through multiple regions or repeatedly pull the same data out to end users are the most common source of unexpected charges here.
In Practice
A media application serving large files directly from S3 to end users across the globe will see data transfer, not compute, become the dominant cost line; a CDN like CloudFront in front of it usually resolves this.