I actually have a fair idea of pricing/locale issues for islands in various parts of the world. I've been talking to real estate brokers who specialize in islands to get a handle on costs, so I'll relay some of the knowledge here.
First, it is really quite easy to buy in island in most parts of the world. Most countries are more than willing to let you buy islands, especially since many of the worlds available islands are owned by third world and developing countries.
I've only looked in the Pacific Ocean, so there may be better deals/locations in other parts of the world.
For an undeveloped resort-class (white sandy beachs, palm trees, plenty of flat land for development) tropical island, the costs vary greatly depending on proximity to other popular resort areas. For an island within a few nautical miles of an island with a significant size town/resort, the costs tend to be around US$50k/acre. However, for an island of equal quality but a bit farther from "popular" (i.e. tourist) areas, the costs quickly drop to about US$5k/acre. Note that the costs have less to do with the proximity to "useful" locations as they do with proximity to "popular" locations.
At the low end (which is the smarter buy IMO), you can buy a nice tropical island with a surface area of roughly a square mile for something like US$4M.
Alternatively, if you move to higher lattitudes, you will find much larger landmasses available at significantly cheaper prices. One location that I have found attractive is the coastal islands off British Columbia. There are many very large islands (25-250 square miles) and countless smaller ones, mostly uninhabited. While technically owned by the state of Alaska, these islands have warmer winters than most of Europe, but cooler summers. The reason these islands may be of interest is because several islands are owned and occupied by various secessionist or unusual groups already so what we are talking about would not likely be noticed. Natural resources are extremely abundant and land is cheap, roughly US$1-5k/acre depending on the specifics (esp. with regards to quantity of timber). Also, you are maybe two hours out of Seattle, and can easily get a fiber link to the North American mainland.
As a side note, when I was last in this region (last August) I learned that several Silicon Valley semiconductor firms have been considering opening fabs on some of these islands because of the availability of large quantities of fresh water (for free!), very accessible shipping facilities, and the potential for large quantities of cheap electricity, while still being in close proximity to a major tech city (Seattle). Whether or not this happens remains to be seen, but it is interesting.
-James Rogers
jamesr@best.com