$100 is the perfect price. It's high enough that nobody's gonna pony it up "for teh lulz", but it's low enough that anybody who's actually serious can get in.
To put it in scale, I work part-time at a grocery store making just above minimum wage, and I make about $100 per week. It's a substantial amount of money, but not enough to get in the way of anyone's dreams.
Plus, it all goes to charity.
To those of you who think Valve should hire screeners: That's exactly what they WERE doing before they instigated Greenlight. And you got mad at them, so they made Greenlight. Make up your damn minds.