At this moment in time we are actually seeing somewhat of a correlation in price to energy content.
Gasoline has about 115,000 BTU/gallon
Diesel has about 138,000 BTU/gallon
(average numbers; you'll see slightly different depending on source; I've seen gas at 125,000 and #2 diesel at 140,000)
That means diesel has about 20% more energy content per gallon, give or take.
Today gasoline around here is at about $3.75 a gallon. If priced according to energy content, we should see about a 20% price differential, with diesel at $4.50 And, give or take, that's kind of what is happening at this moment in time.
Similarly, the energy content of E85 is about 82,000 BTU/gallon, and you should see a price differential down, and we do around here, but it is at such few stations I can't recall what the price is. I did observe recently it was significantly lower in price, and of course yields less MPG.
Whether all this really has anything to do with energy content is anyone's guess. These are just a few facts and observations.