15-20$ could be a price when the game is finished with tons of features. It shouldn't be less than 8$ because then its to cheap, and to put a game up on steam for 100$ then also to upgrade the game engine and servers would cost a lot of money, so they need to earn a bit.
After I checked for more ideas I saw that some people posted some of the ideas I came up with myself before I did, so technically they posted the idea first but I was writing it down before they posted it. In addition, I went more in detail about certain ideas which most of people didn't do, from what I've seen. For example, Underwater adventure, character stats, EXP system, game settings and environment.