When you consider ARM vs AMD vs Intel vs current-gen vs last-gen, AWS sells several 8-core 32-GB machines that have wildly varying performance characteristics.
It used to be that the manner of virtualization impacted performance, too. I think that might be less the case now.
To know if you're getting good value, you really need to try a few different vendors and benchmark then, either with real workloads or synthetic ones.
This is actually a benefit of AWS and other large public clouds. You have a choice due to their size.
If AWS's investment in Gravitron design works well for your workload, it's a ~30% cost savings. They're able to invest in designing and producing the chips due to their size. A company like fly.to is limited in selection and heavy R&D.
The OP was asking what was "most comparable" to Fly's offering. I assumed they meant "what would similar performance cost elsewhere".
By switching from Intel to ARM, they're buying something fundamentally different. Maybe it's a better fit overall! But I didn't understand that to be what they were asking.
I wasn't sure what the dedicated instances for fly.io were running on (can't find it on this page: https://fly.io/docs/about/pricing/#plans). To truly do an apples / apples compare there's networking capabilities, storage, and other considerations also.