Say average power cost is about $0.1 USD / KWh

Say we're using 5850 on a MB, for a total power req. of  500 W.

Calculator at http://bitcoinx.com/profit/index.php says that we'll need an average of 120 days to get a block of 50 BTC.

120*24*0.5*0.1/50 == 2.88 USD / BTC

This is the absolute minimum - cost of "goods", since there is also equipment cost/amortization, building rental costs, support tech. salaries, profit of whoever bothers to mine.

Now, compare that to completely insane valuations that stock market has, like 100 times P/E ratios.

Ergo => current price of 15 USD is only 5 times the very minimal Intrinsic value. And if  KWh costs 0.2 USD, then only 2.5 times, NOT taking other costs into account, and not reflecting the UTILITY of the electronic currency.

Draw your own conclusions.