It's a $900/year subscription if you go the legit route, and not "approved" to use with Hercules. You're supposed to pay ~$4k for zPDT to be fully legit. Thus, most hobbyist use is pirated torrents of ADCD on Hercules.
Everyone in that scene knows that it's not enforceable. Gene Amdahl literally started his own company making plug and software compatible mainframes, and IBM got hit with antitrust rulings trying to shut him down with lawyers.