Elon Musk and his brother Kimbal Musk co-founded the startup Zip2 in 1995. Their father invested $28,000 initially (something Elon now denies). Back then, $28,000 used to be a pretty big amount, given that most venture capital rounds were in the hundreds of thousands, rather than the millions of today. Zip2 was acquired by Compaq for about $307 million over 4 years. The amount Kimbal Musk actually received for the acquisition of Zip2 was much smaller, around $10 million, while Elon Musk received around $18 million.
Elon reinvested that money into founding X, which later merged with Peter Thiel’s startup PayPal. As for what Kimbal Musk did with his $10 million from the Zip2 sale, he used that money to help Elon start his companies Tesla and SolarCity, which explains why he still has a stake in Tesla and SpaceX and risked the rest of the money on his restaurant business. Most people don’t know this, but Kimbal is also a billionaire today.