A somewhat regular sized estate on Maui on the coast goes for ~40 million. On Lanai, I would imagine it to be about the same. Now, think of how many coastal estates are or CAN be on that many acres. It wouldn't take much for just the land alone from those estates to hit 500-600 million.
Now, it is also my understanding that on Hawaii, specifically Maui that almost nobody "owns" their land. They are handed out in 99-year leases from the government (in this case, the "County" of Maui, under the "State" of Hawaii). To give this guy ownership of the land is a huge, huge feat.
Perhaps there are specifics to the land that is not specified or that has yet to be laid out, because 500 mil for that entire island is chump change compared to what it is really worth.
It's pretty common for people to just go out and buy huge amounts of land when they get money, but this really takes the cake.
I don't know whether to be sad or be happy. I'm definitely sad for the people of Hawaii, but I hope he can spruse the place up, make it more energy efficient and environmentally friendly. Maui has been trashed in recent years with only certain areas of it under government control such as La Perouse Bay or Haleakala National Park to be left (relatively) untouched.