No one ever accused Elon Musk of not putting his money where his mouth is."I always invest my own money in the companies that I create. I don't believe in the whole thing of just using other people's money," the eccentric billionaire once said. "I don't think that's right. I'm not going to ask other people to invest in something if I'm not prepared to do so myself." Musk famously dumped all of his personal proceeds from the sale of PayPal -- about $180 million -- into starting Tesla (NASDAQ: TSLA) and SpaceX.