Bill Gates
Bill Gates dropped $31 million to own one of da Vinci's journals. After buying it, however, he worked on digitizing it so that the rest of the world could enjoy it, too, but for a much lower fee of $30.
Nicolas Cage
Nic Cage bought a Tyrannosaurus skull for $276K back in the early aughts. But, as we know, nothing gold can stay, and it turns out the skull had been stolen, and so Cage had to return it to its owner.