He sold his house (which sits on land) for $3.5 million. He thought the house itself (independent of the land) was only worth $500,000, which means that the remaining $3 million must have been for the land. The size of the land in question is only 1/20th of an acre (an archaic measurement of land area still in use in the US, I believe), so on that basis, a whole acre would be worth $60 million - which in his view, is rather a lot of money.