Expected Value = P(success) * Value of success + P(failure) * Value of failure
But you still need calibration in estimating P(success). If you mistakenly think P is too low, you'll pass on the bet. If you mistakenly think P is too high, you'll wager too much $.
But "reputation" is a form of skin in the game (if your audience cares about your accuracy). So verbal probabilities can work, if you lose reputation for being overconfident.