Does the artist that make graffiti can gain income? The awnser is YES. Some people I know started in this word as an illeagal thing. But they started to expand their view of the art to gain money. So they started to make wall paintings for many exclusive places and got paid really well. Then they started to make paintings. They started to get so famous because of their illegal paintings that people know them. So their paintings were very exclusive, so they are expensive paintings.
So, if you are an artist that like graffiti, dont close your mind to only paint on streets. Expand your mind and gain the credit of your work by making it commerciable.