It's all guesswork. They are rich on a scale where money pretty much ceases to exist. They never exchange anything for anything, and they can have whatever they want whenever they want it, so it is all about estimating what the various companies, intellectual properties and ghost milk are/will be worth. Because Tesla makes electric cars, any time there is any kind of interest in the environment Musk will rise because he owns a company that is supposed to be part of the "solution". It's all pretend, and these aren't real numbers in any meaningful sense. I doubt that either one of them could invest 10 billion in anything, because their money doesn't work that way. At that level money is literally just power, since there is literally nothing to spend it on, and the Musk fanboys on Twitter could very well be worth billions of dollars in some estimates.