Server costs? Plex’s serverside only handles auth and verification. Once the client connects to the server, any media is sent peer to peer. There’s no stage where the video goes “to plex” or “from plex”. Saying plex needs to charge a sub fee to make up for bandwidth is like saying qbittorrent should do the same.

Unless you’re talking about the content Plex serves, the ones you have to walk every user of your Plex server through deleting from their apps’ homepage.