The Hidden Cost of Cross-AZ Data Transfer on AWS

Multi-AZ for high availability quietly turned into a network bill bigger than the compute bill — here’s why. Nothing left the region. The bill disagreed anyway. This is one of those “hidden landmines” on AWS: things that never throw a clear error, never show up in a StackOverflow top answer, but can quietly make a system flaky or burn thousands of dollars a month. First up: cross-AZ data transfer. 😱 The assumption that gets everyone Everyone knows data transfer out to the internet costs money. Far fewer people realize that traffic between Availability Zones inside the same Region isn’t free either — a lot of us assume “it’s all internal, so it’s free.”