Elon Musk is well-known today for being the founder of Tesla and SpaceX, as well as being involved with OpenAI, Neuralink, and Hyperloop. But when Musk first started out he setup a web software company called Zip2, which he then sold to Compaq in 1999 for $307 million. That same year he co-founded X.com (and secured the domain name) as an online financial services company. X.com went on to become PayPal.
Musk stopped being CEO of PayPal in 2000 and later received $165 million for his shares in 2002 when eBay acquired PayPal. During this time, Musk also lost ownership of the X.com domain. However, DomainInvesting.com discovered PayPal agreed to sell the domain back to Musk this month, a transaction Musk confirmed yesterday in a tweet.
The sale price is not being shared, with the best comparable price being the sale of Z.com back in 2014 for $6.8 million. I suspect Musk paid a lot more than that and the actual price could eventually be revealed by PayPal. It is a publicly-traded company after all, meaning a future SEC filing may include the details.
As Musk makes clear in his tweet, he has no immediate plans to use the single-letter domain. However, Musk is a serial entrepreneur and I wouldn't bet against him already having an idea or two for how to recoup the cost of X.com with a new product or service in the near future.