What Would It Take to Get the Other 2 of the Big Four on a Boat?

There's going to be some heavy metal on a boat in October 2019, per this link - Megadeth Expand Lineup For Their Megacruise - and this picture:

I see 2 of the Big Four on this cruise: Megadeth & Anthrax.

I am just asking a question. How much $ would it take to get Slayer & Metallica on that cruise? I don't mean to ignite a 'who's a better?' band - because, clearly, I would vote Metallica. The reason this is being tossed around in my brain is because Dave Mustaine has publicly said he would like to do additional Big Four shows. Here's the perfect opportunity to do that. It's his cruise - he can have anyone he wants play . . . which leads to the question I asked above: how much $ would it take to get Slayer and Metallica to join the Megacruise?