Whatever the price, the transaction seems to have been done on friendly terms. Musk's tweet says "Thanks PayPal for allowing me to buy back X.com! No plans right now, but it has great sentimental value to me." It's easy to see why -- the $165 million he earned from PayPal, though small potatoes nowadays, helped him launch SpaceX and Tesla. Those companies are currently valued at around $15 billion and $50 billion, respectively.
X.com has been inactive, but Domain Investing noticed that the Whois had switched from PayPal to "private," and the domain moved from MarkMonitor to GoDaddy. After the site reached out to PayPal, it eventually replied that "we are delighted to sell the domain X.com back to its previous owner, Elon Musk."
The purchase price could come up in a PayPal Securities and Exchange Commission filing if it's in the high seven or even eight figures. As for what Musk will do with it, he's not saying, but this is a guy who dreams up things like Hyperloop in his spare time, so expect something from beyond left field.