Not sure what you're talking about - in aws x86 you pay by core (well as much as pay by core with arm anyways, you can't just buy a 1gb server with 64 cores)
AWS x64 'cores' are the virtual cores you see on hyperthreaded CPUs and map 2:1 to physical cores on the CPU, but the AWS ARM offering doesn't have hyperthreading, so the virtual cores map 1:1 to cpu cores.
You can disable hyperthreading on the x64 instances at the cost of halving the number of cores you have available in the instance that you paid for.
You can disable hyperthreading on the x64 instances at the cost of halving the number of cores you have available in the instance that you paid for.