Stonehenge; ca. 1867
In 1915, Sir Cecil Chubb walked into a property auction in Salisbury and on a whim he bought it as a gift for his wife and walked out £6,600 poorer and the owner of Stonehenge.
Unfortunately, she apparently did not like it so he gifted it to the Crown and today it is estimated the property is worth about $100 million.
(Source)