Let's try all of the above.

For his true-believers, I can't think think of anything that would deter them. He could disect a live baby and eat it on TV and they would still worship him as a pro-life champion.

Perhaps just videos of his press conferences.

His fan club would never turn against him, even if they had him in pictures eating human body parts!

The ransom is not to protect Donald Trump. Whatever they have on the Donald won't be that damaging. It will be released to prove they have sensitive info on Allen Grubman. Depending on how damaging that information is will determine whether he pays. If it weren't something that would put me in jail, I'd release it myself and not give them a dime.

And as I understand it, it was stolen from an American law firm who specializes in celebrities like Madonna et al. So it would have to be something he'd tell them. I bet it's closer to Epstein than Russia.

Edit: Or maybe as was said below, maybe he's really a pauper scamming the world.

I think he would be most embarrassed for folks to find out he isn't a billionaire, thats his ultimate measure of self-worth. What is so horrifying . . . I think if they had video of him doing anything from paying for a hit to taking advantage of really young children, his cult wouldn't stop supporting him. They won't no matter what.