How much money can you make as a amateur board game designer?
If you’re an amateur anything, you make no money. There’s a couple books and perhaps some online guides to presenting ideas to game companies. If you’re serious about making money I’d try contacting a smallish, relatively unknown game company like Real Deal, North Star, or Your Move Games. If they think enough of your ideas they may want to have you work freelance on that game as a project. The bad news is you’d make very little with them. The good news is you’d get your foot in the door and then have a chance to negotiate a better deal on the next game idea.
