Diesel was significant;y cheaper. Always seems that when we have an abundance of something and its cheap, there is a scramble to get into it . Then it goes through the roof and the "shortage " argument rears its head as the reason.
Diesel seemed to me to have increased as more manufacturers began offering diesel, Volks in particular.
Propane seems cheaper after cabs converted their fleets.
There may come a time when we pay the same for all energy based on specific heat content. Or how much power can you get from a particular source of energy. Propane and ethanol produce less energy per unit than gasoline so it should be cheaper per unit.
It's a bit difficult to accurately compare gasoline, diesel, propane and ethanol because each has all kinds of hidden costs and more importantly, financial subsidies. Would we even consider ethanol as a fuel if it wasn't subsidized? Probably not.
Of course if we went to pricing energy based on it's relative energy content, our hydro prices would probably triple.