Update: A friend emails me:
It's clear from the numbers that this market doesn't expect a credible third-party candidate. Interestingly, this means that the reported unconditional probability of becoming president (the third column) is the product of the reported probability of winning a party nomination (one of the first two columns) and the implied conditional probability.
Clinton's implied probability of getting elected conditional on winning the Democratic nomination is 60.3%. That means that, "on average," the Republican nominee has a 39.7% chance of winning. The market's view of the relative strengths of the Republican candidates can then be seen by comparing their implied conditional probabilities to this 39.7% figure: