Yes, Proxmox is free! To be more accurate, Proxmox has several subscription levels, among which the community edition is completely free. One can simply download the Proxmox ISO at no cost and raise a fully functional cluster without missing a single hypervisor feature and without paying anything. The main difference between the paid and community subscription level is that the paid subscription receives updates, which go through additional testing and refinement. In a production cluster with a real workload, it is highly recommended to purchase a subscription from Proxmox or Proxmox resellers.