hmm i can think of better located islands
EG the bahama's
http://www.privateislandsonline.com/islands/bird-cay
http://www.privateislandsonline.com/islands/devils-cay
only $12mill each
Judging by the renderings of the proposed Belle Isle, it looks like he intends to have significant infrastructure there including what are clearly some very large skyscrapers. It could very well be that the islands you posted do not have the proper soil/foundation to support such large infrastructures. They're also only 117 to 250 acres in size (a lot of which looks unusuable, Bird Cay has a large body of water within the island) while Belle is 928 acres in size and located next to a major city making import/export a lot easier.
Belle Isle is probably a pipe dream but the guy has a lot of money and is looking to establish among other things, a new currency. Why re-invent the wheel when he can just use Bitcoin.
Speaking of pipes, how do they plan on handling the shit from 35,000 people a day? Dump it in the river? Not! Build a treatment plant on the island? Don't think so. Pipe it to the nearest city? Bingo! Bonus points if you can name the nearest city. Hint: It starts with a D.
absolutely true.. having a island in a sexy and exotic location free from government rule is a pipe dream.. and now i see the reason why detroit(not exotic or sexy) was chosen .. throw the crap back at them cheaply and easily without it changing the environment noticeably to the natives nearby.