If you have another source of information, feel free to offer it!
I am a little curious why you expect people with less money to being significantly different in their playing habits from those with disposable income. Is there some reason you feel that rich people are more likely to play non-D&D games, and poor people are more likely to play D&D? Looking at prices of games from the time and now, it's not like it's a significantly different different buy-in cost, so it seems you are suggesting an innate correlation between wealth and liking D&D.
I'm going to challenge that assumption, based on anecdotal evidence. In fact, anecdotally, the people buying dwarven forge and flat screen TVs and blinged out gaming equipment that I know of seem to do so more for D&D than for for, say, Fate. But again, this is anecdotal. So I'll stick with the assumption that wealth has at best a weak correlation with roleplaying game preferences. But if anyone has some better metrics (outside of WOTC who I assume has all the metrics, but little incentive to share), I'd love to hear them, being a statistics sort of guy.
EDIT: The "games people typically play" comment seems more plausible. Anecdotally, I know people often go to cons to try new things, so new games, or unusual ones may be over-represented due to that factor. But, also anecdotally, people who's main home game is D&D do seem to play mostly or exclusively D&D at cons, so although it's more plausible, I'm not sure it will be enough of a factor to serious skew conclusions.
It has to do with being a representative sample and the bias that self-selection introduces to the data collection process. It won’t have nearly as random a selection of hobby participants as data collected from, say, a list of 1000 people who bought games from a set of websites or a similar number of people selected entering or leaving game stores.
In my 42 or so years of personal experience in the hobby across 3 states and 5 cities, I’ve only gamed with 2 other people who ever attended an RPG convention...and they went with me. If you expand the selection parameters to include CCG tournaments, I get to about a dozen.
I doubt that my experiences are typical, however. In fact, based on my time at ENWorld, I suspect they’re not.
So while the GenCon crowd may be an actual mirror to the hobby’s general demographics, odds are pretty good that it isn’t.