I don't think Greenlight is really for developers who are small enough to worry about $100. Steam might not make that clear, but it seems as though they are looking to get games like Fez or Super Meat Boy rather than 1000000 or Tiny Wings. It's a way for interesting indie games to be discovered, but Valve likely doesn't want Steam flooded with a bunch of tiny cheap games. There's nothing wrong with Tiny Wings, but I don't want to play games like that on my PC. While paying $100 for a shot on Steam might seem like a bad deal if you are making an iOS sized game, it's not a bad deal if you are making a more complex game that just happens to be made by a small team. If I were making the next Fez or Braid, I'd much rather pay Valve $100 for a chance to be on Steam than pay Microsoft $100 so I can get lost in the mountain of crap that is Xbox Indie Games.