I agree with almost all of what @ambershee said.
I hope it will be something around $2k, maybe a bit more.
Steam could also create a “trash” section where games can be released without a fee, but by default people don’t see games from that section in the regular steam store, so they have to opt into seeing those games.
I also read through what the people in the unity forums think about this (there’s the same thread like here), and actually they are a lot more positive about it. Probably because Unity people are burned a lot more by the bad reputation their engine gets from all those trash games on steam.