A paid plex share is a plex server that someone is running + selling access too.
This is against plex' terms, gets plex accounts banned; and in some cases, Plex (co) has taken rather drastic action by blocking entire VPS providers from reaching plex.tv; thus plex server software no longer functions on those VPS's at all.
Naturally, people selling shares want to maximize profit, so they use VPS providers on the cheaper end; resulting in cheaper VPS solutions being blocked for everyone.