I'm probably going to regret trying to formulate a serious response, but here goes.
The only way to make sense out of this question is to think more generally about the meaning of comparisons across academic and non-academic fields. I can suggest some metrics:
<ul type="square">[*]time spent attaining mastery[*]exclusivity - i.e. how many people on Earth have attained a certain level of mastery [*]----> But even across academic disciplines this item opens up a whole host of other ambiguities. Are we talking about mastery of the role of women in the economic history of medieval France, or mastery of economics? It's not at all clear to me that the scope in which one declares oneself an "expert" is anything but arbitrary[*]What's the average performance metric (for poker, winrate) among enthusiasts who have actually attained a PhD in another field[*]---->But this is also messy. Chris Ferguson may have a math PhD, but there's no way of knowing how much of his "energy" or "smarts" he apportions to math versus poker. And at any rate, suppose he's just so brilliant that his tournament winrate reflects far more than a "PhD in poker". How would you find the lower bound of PhD equivalence?[/list]
I'm sure there are other ways to look at the question I haven't thought of.
I'm not going to poo poo your question because it got me to think about the difficulties inherent in attempting to credential expertise. But making meaningful sense of questions like "What's the equivalent of a masters in soccer?" or "What's the equivalent of a bachelors, only in chess?" is a thorny problem not confined to poker.
I was tempted to violate my moratorium on poo-pooing, but I edited this to keep myself in check.