Not every individual person would require this basic income. Infants, children, or those under care of others, etc. And I don't think Zuckerberg is advocating guys like him are paid a UBI. In the speech he mentions "people like me" should pay for some of these things he's discussing. So you're math is probably off - not that this wouldn't be extremely costly, don't get me wrong, but you're probably looking at maybe 200 million working age people. If you also take him at face value that "people like him" should pay for these things, I also imagine anybody who is gainfully employed and making it on their own wouldn't qualify. So that number likely drops significantly.