Gangs use graffiti to mark territory and presence. They don't necessarily steal it. The person writing it is designated to write it. Overall, they don't use much paint and probably don't care about paint quality.
Artistic tagging and graffiti are a different thing. Due to the amount of paint used, they do tend to steal it if they lack money. "Racking" or stealing it is also considered a part of that outlaw culture. A large mural can cost hundreds of dollars to execute. Even a small picture can take a few cans of paint.
To get an idea of the amount of paint, search for images of "gang graffiti", and then for "graffiti murals".
Artistic tagging and graffiti are a different thing. Due to the amount of paint used, they do tend to steal it if they lack money. "Racking" or stealing it is also considered a part of that outlaw culture. A large mural can cost hundreds of dollars to execute. Even a small picture can take a few cans of paint.
To get an idea of the amount of paint, search for images of "gang graffiti", and then for "graffiti murals".