When I've heard Elon talk about this before, his idea is that people interested in moving to Mars would actually sell their house and Earthly possessions in order to pay for the ride. They won't be needing all that junk anymore anyway. Once you're prepared to sell all of your assets, then having a net worth of $500k is essentially the same as being able to afford $500k.
You're assuming nobody else wants to go to Mars, and you can actually offload all your junk before the market is flooded with the crap everyone else is trying to pawn off to leave this planet.
I don't think the crowd willing to sell all their assets and buy a one-way ticket to a planet where the odds of dying of old age are astonishingly small is large enough to collapse an economy.
Don't get me wrong - I'd love to go to Mars myself, but I wouldn't like to make such a long-term commitment without a good chance not to die there. And that doesn't even include my family.
I doubt it would collapse the economy, but it's worth keeping in mind that if you're actually planning to go, you might want to beat the rush when it comes to getting the money together.
