I have no idea how the evil Empire collected taxes a long time ago, in a galaxy far, far away. (I suspect that R2D2 kept awesome records in case he was audited; Darth Vader hid his money on Endor, a forest moon bearing a striking resemblance to the Cayman Islands; and Chewbacca never bothered to file at all.) But here in the U.S., gains from the sale of a business are treated as capital gains and subject to tax up to 15%. Lucas is taking half of his proceeds in Disney stock, so that part escapes tax for now. (He'll pay if he sells those Disney shares sometime down the road.) But that still leaves up to $2 billion in fully taxable cash gains. And that means up to $300 million in tax for Uncle Sam.
At least, that's how it works this year. On January 1, the Empire strikes back, when those Bush-era rates expire. Unless Washington gives us a new hope, that capital gains rate jumps to 20%. President Obama has said he wants to extend the current rates for income under $200,000 ($250,000 for joint filers), and the Senate has passed a bill to do just that. But if the 20% Clinton capital gains rate returns, at least for guys in Lucas's bracket, selling in 2013 could have cost him up to $100 million more in immediate tax. That's at least enough to recondition a Millenium Falcon or two!