This amount of money is insanely high because the whole revenue during 2009 and 2010 was about $100 million per year only. If you realize that there is only a tiny profit margin if any (profit is at most a small fraction of the revenue), you see that you would clearly need many decades to repay the investment. It was never going to be repaid because the company was never competitive.
The U.S. taxpayers may have lost half a billion but this loss may have good consequences, too. Tens of millions of people may finally see the light and realize that everyone who talks about the imminent demise of carbon-based energy is a crackpot who can make you completely broke within a year.