I got some mail from Engineuity explaining that they had worked out the cost and weight ratio of their system using Aluminum rather than Magnesium. Reworking the calculations, we can see that their claim that the Aluminum would weigh about 3 times as much as the gasoline equivalent is correct, but the idea that Al is about as expensive a source of energy as gasoline is pie in the sky.
1 gallon of gas is 130.88 mJ.
1 mole of H2 is 285.83 kJ, so a gallon of gasoline carries the energy of 457.89 moles of H2. The reaction
3H2O + 2Al -> Al2O3 + 3H2
yields 1.5 moles of H2 per mole of Al, so 457.89 moles of H2 would require 305.26 moles of Al. The atomic weight of Al is 26.981538, so that would be 8236.5 grams of Al. With gasoline weighing about 2.8 kG per gallon, that's about 3x the weight of Aluminum. Aluminum is a couple of dollars a kilo, so to get a gallon of gas equivalent from Aluminum would cost $17 per gallon plus any retail markup and tax.
Engineuity says they expect the recycling value of the oxide would close the gap between the cost of Aluminum and gasoline. But alumina sells for about $400 per metric ton. A kilo of Al, which costs about $2.20, would yield about 1.9 kilos of oxide which is worth about 75 cents. Recycling can offset the cost of the Engineuity process by at most 35%, which would still cost about $11 for the Aluminum equivalent of a gallon of gasoline.