I believe that the trick is not to show the developers the bill.
I haven't had access to the AWS bill in 4 of 5 companies that I've worked at. Why? Fuck if I knew, but I got vague answers like security and compliance when I asked.
I wonder how many thousands could have been saved if all devs could see what they're actually paying for but not using.
I think it's a combination of things. My experience definitely parallels yours: when developers have access to the bill they tend to realize the cost of the services they are using. Sometimes even resulting in optimizations to those costs.
At the same time AWS can get fucked with how horrible their bills are to understand. They don't exactly go out of their way or even slightly on a good path to deliver a clear bill.
So even if the developers have access to the bills they might just end up with an impenetrable list of bullshit from AWS
They show us the cloud bills at my work. It's a sobering moment when you see the monthly price tag for your team's development work.
I'm not saying it's not justified or that the company doesn't make up for that cost many times over but it's very useful to have a real world reference.