Although a complete cost analysis is much more complicated, the particular question only requires the following: an energy cost estimate (in terms of $/Joule). This was directed to be done by taking volumetric or mass based costs of each fuel as well as the associated energy conent or energy density. I am finding it very difficult to find any reliable information through google. Where I'm living gas costs are about $1.02/L right now. My research lead me to an energy density of 32 MJ/L for octace which I used for the gasoline calc. For Hydrogen, most costs seems to be between 3-4 $/kg with an energy density of around 120 MJ/kg. These numbers result in similar $/Joule results for both fuels, which I'm pretty sure should not be the case. Do those numbers make sense? Is there a reliable source where I can check my data. Please ignore all other costs and efficiencies associated with the two fuels and engines. This is just a very simple calc once I have the correct data. Your help is much appreciated.