AWS in particular is handsomely engineered so you can't answer this sort of question, as well as making people spend more without need.
- Per resource cost is turned off by default. Turning it on costs money. How much? ¯\_(ツ)_/¯
- Services like S3 will bill you per API call. How do you know who is calling your APIs? Using Cloud trail, which is turned off by default. Turning it on for this sort of analysis might cost a lot $$$ for a reasonably used bucket.
- CloudWatch will bill you per custom metric and adds up quickly. Why do I have so many custom metrics? Good luck finding out.
- Many costs are tied with stuff that's really hard to control e.g. egress traffic.
- Using many accounts is by far the best way to segregate unrelated projects. But then, you will end up paying for many criminally underutilized NAT GWs, VPN connections, Load Balancers, etc...
- Many services are ha, multi az only. That makes AWS unnecessarily expensive for dev environments / non critical services.
The list goes on. I know a fair number of consulting companies making the money of their lives reducing AWS bills.