They are also storing the data in S3 (ingest bandwidth, storage). Plus running an RDS instance (instance cost, x2 if replicated, bandwidth again possibly x2 or more, IO cost) as well as local storage (EBS size/IO cost). And the size of the raw metrics might not be the wire size, especially if it’s uncompressed JSON. My guess on reviewing the post is that a lot of their savings were inter-AZ bandwidth conservation. But it’s hard to say without poking around in their AWS console :)