If they do offer appearance fees, it's probably written in the contract that they can't disclose that information to anybody. I wouldn't be surprised if they paid somebody like Yul to come back for WaW. He seemed to have no interest at all. And I wouldn't be surprised with someone like Parvati, either.The only one I recall hearing about was doubling of all prizes for All Stars other than the winner amount.
I wouldn't be surprised for something similar for WaW. I think I read they were guaranteed a fee for the reunion. Don't know how that differs. EoE also meant all players would get paid a certain amount for appearing in each episode.
I'm not sure about Australian Survivor, but US Survivor has a good problem... they have an abundance of people who are dying to return. They'll never have a problem finding enough people, so appearance fees don't make a ton of sense from a business perspective.