How much does a game selling here make on average a month? (Steam has a difficult process and ridiculous fees to get a game on their site).
There's a wide range of content on the site, and we host mostly free games. The average earnings for a project isn't going to be meaningful. The kind of game you make matters: If you make a game that people aren't into then you won't make much, but if you make something engaging then you could make thousands, tens of thousands, or hundreds of thousands. (We've yet to have a game make over 1 million dollars though!)
If you want to make money, make a typical waifu-collection gamblegacha game like most of the rest of the market has drifted towards. Hire an artist who can make cute girls and rake in the easy money. Only thing is you'd want to hire good artists and take some time to program it decently-enough to work on a wide variety of phones. Or just make it iphone-exclusive and not worry about any of the Android fractured userbase.