You're wildly overestimating the number of clubs that are set in the halves. Off the top of my head, the Sharks, Dragons, Bulldogs, Titans, Souths and Knights all have reason not to be totally happy with their starting halves and several others have absolutely no depth. $500k is nothing for a starting half nowadays and wouldn't be a disaster for a backup.
In a world where Drew Hutchison, Matt Burton (as a 5/8), Tyson Gamble, Kyle Flanagan, Dean Hawkins, Lachlan Ilias, Tanah Boyd, the reanimated corpse of Kieran Foran, Jamal Fogarty, Ethan Strange, Braydon Trindall and Blaize Talagi (as a 5/8) are all getting starts, we're not going to struggle to find someone to take a punt on Sullivan.