[Rupert]

A few hundred million credits (rather than the MCr45-50 of a standard 200 DTon yacht) should buy the mega-rich something around 500 DTons, assuming it costs as much as a warship (and therefore that it's fairly high performance). A billion credits should buy 1,000-4,000 DTons, depending on performance, how much of the 'armour' is gold or iridium, and so on.