For one, there are almost(?) no unclaimed patches of land available. If you look at a political map of the Pacific, most of it is locked up between the EEZ of various states and micro-states.

http://www.spc.int/aquaculture/images/stories/dfatpacific%20...

Most of these countries are also very poor (Africa levels) and heavy recipients of foreign aid. So politically, they are corrupt and inaccessible.

So the best bet is to build an oil-rig styled structure or a large ocean-going ship. However, both options go into the hundreds of millions to the billions; a super-rich individual is naturally mobile, so he doesn't need a fixed patch of land that he politically rules.

That said, you may be interested in this: http://en.wikipedia.org/wiki/Seasteading