Spray paint graffiti generally isn't used by independent criminals. It's usually gang related. Or used by an artist (But they'd spring for the good stuff, and aren't really doing any harm).
You can get spray paint for $1 at the dollar store. It's crap, but it's not like you're using to rust-proof your patio furniture. You can even get "used" cans at some places (like pawn shops). Considering the age restriction on purchasing it, I'm guessing either A) people turn a blind eye to that rule B) it gets shoplifted, or C) Old gang member member buys and distributes it.
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".
My point is this: people are, essentially, paid not to vandalize. Don't buy paint and pocket that money for being a good citizen. Yet they go and do it anyways.
The idea that they will suddenly change their lifestyle after being given some unconditional stipend seems to be in a direct contradiction with the observable behavior of vandals.
You can get spray paint for $1 at the dollar store. It's crap, but it's not like you're using to rust-proof your patio furniture. You can even get "used" cans at some places (like pawn shops). Considering the age restriction on purchasing it, I'm guessing either A) people turn a blind eye to that rule B) it gets shoplifted, or C) Old gang member member buys and distributes it.