I've read a lot of other people's AWS invoices. Different companies, different products, different teams. The waste is nearly always hiding in the same five places, which is either comforting or depressing depending on how you look at it. Here's the list, so you can check yours before paying anyone to do it.
1. Compute sized for a day that never comes
Instances get sized at launch for the traffic everyone hoped for, then never revisited. CPU sits at 8% for a year while the invoice quietly charges you for the other 92. Pull two weeks of utilisation metrics on your biggest instances. If peak usage doesn't get near half of what you're paying for, you've found your first saving, and it's usually the largest one.
2. Non-production environments that never sleep
Your staging environment does not need to exist at 3am on a Sunday. Neither does the demo environment for the customer who churned last year. Anything non-production running 24/7 is paying a 4x premium over business-hours scheduling, and the fix is an afternoon of automation.
3. Storage nobody has looked at since 2023
Unattached EBS volumes from long-dead instances. Snapshots kept "just in case", hundreds of them, of those same dead volumes. S3 buckets full of logs in Standard storage that belong in Glacier or in the bin. Storage is cheap per gigabyte, which is exactly why nobody audits it, and why it compounds into real money.
4. The NAT gateway processing fee
The line every founder squints at. NAT gateways charge per gigabyte processed, and a chatty workload in a private subnet can rack up more in processing fees than the infrastructure it's talking to costs. VPC endpoints for S3 and DynamoDB are close to free and often wipe most of this line out.
5. Data transfer, the mystery meat
Cross-AZ chatter between services that could live together. Traffic leaving AWS that could be cached at the edge. Replication you configured once and forgot. Nobody can predict this line from first principles, but it rewards twenty minutes with Cost Explorer's filters more than any other.
The uncomfortable summary
None of these require genius. They require someone actually owning the bill, which at most startups is nobody. That's the real finding, and fixing it matters more than any single line item.
Want yours read properly? The cost audit does all five of these and prices every finding in dollars. Or book a free half-hour review and I'll tell you what I'd expect to find.