As billionaires and millionaires flee one time tax havens like Switzerland, Monaco is preparing to welcome them with open arms. Luxury housing in the French Riviera is booming right now and if you have any doubts about that, then take a look at this $400 million 35,500 square-foot penthouse atop The Tour Odeon on Monaco’s Mediterranean seafront that some believe will sell for much more making it the most expensive penthouse on the planet.