Celebrities get paid to write their names on things, because the world is hardly fair and rarely makes sense. However, these people discovered ways that are nothing short of brilliant to squeeze every last dime out of their signatures.
Salvador Dali famously loved his goddamned money (in fact, he is only slightly more well-known for being a painter). There is a long-standing rumor that he used to doodle sketches on the backs of his personal checks to keep people from cashing them, which we desperately hope is true, because it is hilarious.
Later on in his life, when he was no longer able to produce any artwork, Dali would sit and sign his name to literally thousands of blank sheets of lithograph paper. He would then sell the blank sheets for about $40 each, single-handedly funding a vast network of Dali forgeries that art dealers are still cutting through like a vast thicket of pirated Eraserhead DVDs 50 years later. The best part is, Dali knew that this was precisely what his blank signatures would be used for -- he just didn't give one handlebar-mustached pigshit, because he made hundreds of thousands of dollars by doing it (see "loved money more than painting," above).
Pablo Picasso, arguably one of the most famous artists in history, was not above trading an autographed doodle of a wiener dog for a meal.
He wouldn't do it for just anyone, though -- when the proprietor of a smaller eatery asked him if he wouldn't mind sketching something on a napkin, Picasso replied that "he only wanted to pay his bill, not buy the restaurant" (because when your name is that valuable, you aren't just going to exchange it for some fucking sandwich). He once sat at a bistro, doodled over the entire tablecloth, and slowly destroyed it before he left, because Picasso apparently loved screwing with people in restaurants.
Baseball legend Joe DiMaggio was able to earn more money signing things than he ever did from actually playing the sport.
This worked because DiMaggio was remarkably shrewd when it came to his autograph. He'd happily sign a ball or piece of paper for anyone who asked, but he absolutely refused to sign bats except at special events. That made a DiMaggio-autographed bat extremely rare, allowing him to charge whatever the hell he wanted to sign one (generally a few hundred thousand dollars per appearance). We assume Joe used one such bat to tattoo Paul Simon for not paying for the use of his name in "Mrs. Robinson."
Neil Armstrong will be forever remembered as the first man to walk on the moon (and is likely the only person to do so that any of us can actually name). However, back in the 1960s, he was just an astronaut earning nowhere near enough money to pay for the life insurance he needed to support his family in the (very) likely event that he and his crewmates got superkilled by a space explosion.
While locked in isolation for a month leading up to their mission, Armstrong and his crew came up with the most brilliantly cynical idea in the history of time -- they signed hundreds of autographs. Anything space-related they could find, they signed, and then had the items sent to a friend to be dropped in the mail on the exact date of their moon mission, July 16, 1969. That way, in case they all died in the callous vacuum of space, their families would be able to sell the bejeebus out of their autographs (which would be postmarked the very day of their spectacular deaths). Because nothing opens wallets quite like tragedy.