In 1995, Elon Musk and his brother Kimbal founded Zip2 with $28,000 out of their father's bank account. 4 years later, it was acquired by Compaq for $307 million, with Elon making $22 million. A little under a month later, he put $10 million of that into co-founding
X.com, which he envisioned to be the future of online banking. A year after, it merged with Confinity to create PayPal.
In June 2002, Elon Musk founded SpaceX using some of the $12 million he had left over from Zip2. Then in October, PayPal was acquired for $1.5 billion, and, being the company's largest shareholder, he made $165 million. With this large sum of money, Elon later allocated about $90 million to SpaceX, which could have gone to waste had he not taken a crucial investment from Draper Fisher Jurveston in 2008.
In 2006, he became a significant investor who jumpstarted his cousins' company Solar City, that of which he is currently chairman. From 2003-2009, he had spent over $70 million of his own money to ensure the longevity of Tesla Motors. For a time, he was chairman, lead investor, and head designer, but wasn't CEO until 2008, when they nearly spiraled into bankruptcy after missing out on a $100 million investment.
These companies, who each had their fair share of financial crises, are now worth billions of dollars today, and will only continue to flourish. Elon holds about a 20-30% stake in Tesla, Solar City, and SpaceX, all contributing to his estimated net worth of $13.2 billion.