He paid probably a not too large sum to obtain it in the early 90s. Then it became ownership of PayPal due to the merger. And he bought it back for undisclosed amount in 2007 from PayPal (now eBay). He may have paid a million or a few, who knows.
I think he registered it in 1992 and so possibly paid nothing, or maybe $35?
(There are stories going into further detail which I haven't read.)
I believe that this domain is worth more than ten million US dollars.