A 6 lb gold nugget found in Butte County, California, sells for around $400,000, around 4 times it's actual gold value (nuggets have a higher "curiosity" value and so sell for more"). 

Links: http://www.latimes.com/local/lanow/la-me-ln-gold-nugget-sells-anonymous-buyer-20141027-story.html & http://www.foxnews.com/us/2014/10/26/giant-gold-nugget-found-in-california-finds-secret-buyer/

- Thanks Ryan for that one

Note how the seller is keeping the location & his identity a secret. Smart man.