The most expensive house in the world is worth $1 billion and is in Mumbai, India. It belongs to energy billionnaire Mukesh Ambani. However, it was built specially for him (including a cinema, helipad and other little luxuries) so perhaps it wasn't sold in the usual sense. In fact most of the most expensive homes in the world were built specially for their owners.

For a house that actually changes hands, you can look at the Villa Leopolda in France, on the Cote d'Azur. This was built in 1902 for King Leopold of Belgium, and now belongs to Lily Safra, widow of the banker Edmond Safra. This is worth $525 - it should have been sold a couple of years ago, to a Russian businessman, but the sale never went through. Maybe he found a cheaper one - he could hardly fail to do that, as after the Mumbai house this villa is the world's most exensive private property.