AWS ACM has always been free. ACM has never allowed you to export private keys. However ACM does have a way[1] for you to use ACM keys with EC2 instances: Nitro Enclaves. Nitro Enclaves carve off a little piece of your EC2 instance (memory + VCPU) into an isolated VM that feels a little bit like an HSM or a secure enclave.

[1] https://docs.aws.amazon.com/enclaves/latest/user/nitro-encla...