Why is the $100 fee a good decision? Why not a lower value?
Originally, uploading games to Greenlight was free. This meant that there were a lot of troll submissions, or people who misunderstood the service for a wishlist. There were also many very sub-par entries, either very early in concept stage, or done by amateur teams. The harsh reality is that most indie games are terrible, precisely because most of them are made by inexperienced teams. The harsher reality is that those don’t deserve to be on Steam, and their presence there was harming everyone else.
The $100 fee makes sure that not only legitimate developers submit their games, but it also discourages people with weak ideas from doing so. A lower value, say $10, could easily have blocked most of the trolls and confused users, but it wouldn’t stop the unmarketable games from being added there. With this relatively high fee, you need to have SOME faith that your game has a decent chance of being approved. Again, remember that Valve’s business is not to include EVERY indie game on Earth, merely the best of them. People who are amongst those almost certainly already have a following around them, and have built enough confidence that this is something that people would pay money for. From that view, $100 becomes a very irrelevant value – it’s the value of a few dozen sales even at low prices. If you don’t think that your game will sell thousands of copies, then don’t bother putting it there.
What about poor indie developers who can’t afford $100?
This is where things got really ugly on Twitter. There were many developers claiming that they simply can’t afford the $100 needed for the submission. They dedicate their lives to the craft and live on the edge, without a cent to spare. But again, this only seems relevant if you have a minor game that nobody has heard of. If you have a fanbase of thousands of people who want to play your game, surely you can do a fundraiser asking for 100 dollars for that purpose. Once your game is on Steam, you can just give them Steam keys (which apparently you can do at will), so this also works as a Kickstarter/Pre-order model. Or you can ask somebody else to cover that cost for you – if you truly have a Steam-quality game on your hands, somebody will pay that for you. I even saw some Tweets of people offering to do so. You’re only “left out” if you’re simultaneously extremely broke and unpopular – and, at that point, you’re not going to be successful in Steam anyway.
Finally, if your game is popular enough to eventually pass Greenlight, it should be popular enough to net you well over $100 of sales on OTHER, more indie-friendly websites, such as Desura. Once your game sold a few copies there, use the money to pay your entry into Greenlight.
Remember: Greenlight and Steam are not your personal marketing page! They are a STORE, to which you will only be admitted after proving that you can reach their standards. They owe you no favours.