Cost reviews are usually run as procurement exercises. Read as engineering documents they are far more interesting.

What the line items reveal

Egress charges show you where your data actually flows, which is often not where the diagram says. A large NAT gateway bill means services are talking to the internet you thought were internal. Cross-zone traffic that dwarfs your application cost means chatty services that should have been one service.

We once found a service that cost more in inter-zone transfer than in compute. The cause was a cache that had been placed in a different availability zone from every one of its callers. Nobody could see it in a diagram; it was obvious in the bill.