Trump currently has the lowest payout. Pick any of the dems and if they win and I get more than if I bet on Trump today and he won. Am I wrong?
You're exactly right. But that's a very different bet then the subject line: does Trump get re-elected? Because the odds include the odds of the Dems all competing against each other in addition to competing against Trump .
A Final Four analogy would be having to pick a title winner after the first semifinal game, but before the second. Let's say Michigan State wins the first semi, but Virginia and Auburn haven't play yet . If you had to pick the team with the best odds to win the title, it'd be Michigan State because they only have to play one game to win, while Auburn and Virginia would have to win two games to win.
That's what it's like with Trump. He's already won his semifinal game (effectively). But the Democrats all have to play each other before they can play Trump. And that's all very uncertain. There's no clear frontrunner, so the odds of any one Democrat aren't that great. But once a primary winner emerges, their odds of winning the Presidency will skyrocket.