Nitpicking Avatar: The High Cost of Unobtanium

I already did a post on Avatar, noting among other things the amount of work that went into choosing the name "unobtanium", but apparently I missed a significant hole in the plot.

In this article at Space.com, Seti Institute scientist Dr. Seth Shostak explains that the estimated cost of transporting unobtanium across the stars far outweighs its value.  From the article:
"Now let's consider the tariff for sending a kilo of unobtainium back to Earth. Our descendents in this film have some pretty nifty looking rockets, and we hear shortly after the opening titles that the trip to Pandora takes only about five years (as measured on-board). Well, even the nearest other star system, Proxima Centauri, is 4.3 light-years from where you're sitting. That means that transport between Earth and Pandora occurs at 85% the speed of light or more!
Getting a kilogram of unobtainium (or anything else) up to that speed, and then decelerating it at the end of the ride, takes at least 1017 joules of energy. That's freshman physics. What's the cost of that energy? Our cheapest joules are supplied by your local utility company at about ten cents a kilowatt hour, or 36 million joules per dollar. At that rate, the price of shipping a kilo of unobtainium works out to $3 billion, or — assuming  2% annual inflation between now and 150 years from now — $50 billion in 2154 c.e. dollars (that's the year in which the film takes place)"
 One counter argument might be that by the year 2154 we have developed a much cheaper energy source and can therefore travel between stars at a reduced cost.  But, if we already have cheap energy, why are we fighting tooth and nail to obtain unobtanium?  It is a very interesting observation, something I wish I had noticed before.

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