It's not an illusion; David Copperfield really is a billionaire, and it would take a lot of time and effort to make that amount of money disappear. His rise through the ranks of the secretive Magic Circle is astonishing; his grandfather taught him his first card trick as a child. By age 12 he was a member of the Society of American Magicians, and already performing on stage. At 16 he was so skilled he was actually coaching and training other people to perform magic, and before he was 20 he had a residence in Hawaii. From making the Statue of Liberty disappear to walking through the Great Wall of China, there's seemingly nothing Copperfield can't do.
For years he's been the most in-demand magical performer in the world, and so he can charge what he wants. That wealth has enabled him to acquire the most valuable collection of magic memorabilia in the world, featuring priceless artifacts from Houdini and Georges Melies.