I earned more than $500 a month once: https://www.kickstarter.com/projects/lyoc/last-year-of-carbo...

This game was developed by my friends and I during college, then we Kickstarted a few years after.

Sales have dwindled since, but I still like the game. So much so that I turned it into a free web app. Still a WIP but it's maybe 75/80% there.

Play for free here, you will need to make an account: https://lastyearofcarbon.com

Buy a copy here, but you should play the game first to see if you like it: https://lyoc.shop