Owner: Lily Safra, worth $1.2 billion
Price: 500 million euro ($750 million at the time) in 2008
King Leopold II reportedly built a series of waterside homes for his many mistresses. This 20-acre estate was valued at 500 million euros in 2008, when Russian billionaire Mikhail Prokhorov attempted to buy it. He eventually pulled out of the deal, forfeiting a 50 million euro deposit.