$600 is tremendous for most people in the US right now – and it doesn’t really matter why the cost is that high, only that the cost is that high given the aim of the festival.
If the figures from the census are self-reported, in a situation like Burning Man, I would question their accuracy. It’s possible that no one has reliable data, but all data should always be treated with a similar vigor. Obviously everyone who goes to BM thinks they are making a statement, but the people who would benefit the most from such a statement likely can’t afford a $600 ticket.
So I guess the tl;dr from me is: I don’t think there is evidence that BM is the populist movement it wants to be anymore.