Interesting article on Penny Arcade about this whole fee thing;
http://penny-arcade.com/report/editorial-article/the-100-fee-for-steam-greenlight-submissions-is-exclusionary-and-wrong1
Regardless, I honestly do think it is too much to ask for $100 for developers just to weed out trolls (I'd even say just $20 would suffice). This gesture is less about money for the developers, and more about giving everyone a fair chance to properly promote their game. You're going to get trolls no matter what, any amount of money would deter them, but setting the bar arbitrarily high just alienates those who just want to make a simple game in their spare time and hopefully share it to the world. For some, yes, $100 is too expensive for them, as not everyone has a million dollar budget, or even a $10,000 budget. I'd imagine plenty of college kids would love to utilize this service to promote their games but will be locked out due to this cost restraint. This service is less about just getting the top-tier, high quality games, and more about letting the community get the games they want, and the fewer the barriers the better.
Remember, VALVe doesn't need this money. Everything is going to charity, so this cannot be considered "advertising" or some other arbitrary thing that might need money because it is completely non-profit for both sides. In fact, I would rather that they take a small percentage for "Licensing costs" or "server space" as that would justify a larger cost in the first place, placing it back to a business dealing. VALVe doesn't need any of this money, so why make it this arbitrarily high? Because this is simply a deterrent for illegitimate contributors, and because it is simply for charity and not an actual business transaction, it essentially punishes the legitimate ones for something that could easily be solved by a review process beforehand or just a smaller fee. $100 doesn't seem like much in terms of large-scale game development, but for what this service is truly aimed at, it is a larger barrier than it needs to be.