He has money from the profits after cornering the Top Ramen market. They get small payments for doing chores. Some relatives might give him money too...He could sell his signature. etc.
Weren't some of the guards getting Manson's artwork and selling it on e-bay? Probably a good investment.
Over here, I think he wouldn't be able to do even that: the law's very strict about not allowing people to profit financially (even indirectly) from any crimes they've committed, and it would be held that the value of his signature rests on fame/notoriety achieved as a result of his criminal convictions and their publicity.
Can't wait for the movie. Will be sure to order a hot chocolate at the concessions stand before it starts.