Bets with money assume fairly constant and universal utility/$ rate. But that can't be assumed in this case since money isn't worth nearly as much if the world is about to end.

So you'd have to adjust for that. And of course even if you can figure out a fair wager given this issue it won't be equivalent to the right degree of belief.

It is harder to recover probabilities from the bets of this form that people are willing to make, because interest rates are a confounding factor.

It isn't that hard, is it? We just find the interest rate on the amount B got to begin with, right?