At first I thought it was a little steep, but this way I feel like it motivates the serious teams just as much as it keeps out the trash.
With $100 invested up front, it gives you an extra incentive to keep working and not just abandon the project for months/years at a time like so many teams do.
Yes, what people who have given hundreds/thousands of hours of their unpaid (spare) time into a project really need is (a) $100 (raffle ticket to the court of public voting) on the line to keep them motivated. That's the key!
From my perspective, putting the money down might seem like more of a tangible investment than spending time on the project, depending on how much work I had put in. I'm not saying it would matter to everyone, but every project and team is different.