ACM costs with using subordinate CA certificate whose parent authority is externally hosted.

I am needing to understand the costs associated with Installing a subordinate CA certificate whose parent authority is externally hosted. Does this bypass both the $400/month (long duration certificates) and $50/month (short duration certificates) costs? If so then would we only be paying the cost for the certificates that are issued this way or does that cost go away as well? Basically I am wanting to use ACM for implementing IAM Roles Anywhere for a PKI strategy on internal endpoints. We have our own Private CAs so we don't really have a need for AWS to do it and would like to avoid the additional costs. I am also not opposed to using OpenSSL or another option.
Is there any detailed information for setting this up? It seems like all the documentation I find is for ACM PCA hosted certificates.