Someone analyzing cloud cost for the first time almost always focuses only on EC2 and S3 storage charges. Then they see the "Data Transfer" line on the bill and get a shock — in some workloads, data transfer accounts for more than 30% of total cost. What's even more baffling is that it's not intuitively visible where this cost comes from. Two EC2 instances merely exchanged data, yet a per-GB charge is applied for the sole reason that the two were in different Availability Zones (AZs). Data transfer cost is the direct result of AWS's network's physical structure — regions, AZs, the internet boundary — being reflected right onto the price sheet, and you can only reduce it once you can picture this geography in your head.