There are a couple ways to do it.
1. In game ads. When I was at MochiMedia, the top game developers earned around $10k/month, so it is possible. That said, there were only a handful that were able to do that well. Most people made far less. Even games that you would consider very successful made around $200-$400/month. The money to developers is around a $0.25 CPM. That means to make $250 in a month you need 1MM game plays.
2. Virtual Goods. Since I left Mochi, they started their MochiCoins program where developers can sell virtual goods directly to game players. I don't know the official numbers, but I heard the successful guys were doing as much as $1k/day. Again, that's the top end and most people were far lower. The bar is much higher in creating a good game here as you're going to try and get a user to actually pay for it.