The ignored externalities are massive, more than comparable to the current price.

"A 2009 study by the Environmental Law Institute[5] assessed the size and structure of U.S. energy subsidies over the 2002–2008 period. The study estimated that subsidies to fossil-fuel based sources amounted to approximately $72 billion over this period and subsidies to renewable fuel sources totaled $29 billion." They give a breakdown of why some aren't true subsidies here: http://en.wikipedia.org/wiki/Energy_subsidies

Counting per megawatt avoids looking at the subsidies that have been amassing to fossil fuels over decades, and completely avoids the issue of externalities.

Deepwater Horizon is but the most recent example of a catastrophic spill. We never hear of a wind spill. In any case, the big externality is being allowed to dump so much CO2 in our shared atmosphere.

As has happened in other countries, the subsidies have gone down over time. With increased growth we get faster drops in price (that experience curve, again). Compare this to the regulatory capture from oil and gas that has led to ever higher subsidies.