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How do you know that it "costs" the same 128 kcal to reverse the reaction?
It may cost more or less, with some heat coming or leaving, for example.
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The 118kcal/mole of energy expended to dissociate water (or the 118kcal/mol released when hydrogen is burned) is something that God determined when He set the laws of universe. It's kinda like gravity... it just is. That said, you will always have to expend a little bit more than that to pay Entropy (he's the man who invented friction and other energy taxes). If you have a very good process, you don't pay him much. If your process sucks, you'll pay a lot. But you will never pay less than 118kcal/mol; God says so.
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Think this: when water change its state from liquid to vapor, it takes heat from the environment. What happen in the opposite direction?
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Umm... condensation? Seriously, though, the latent heat of vaporization (thermal energy required to boil water or released during condensation) is the same in both directions. Most thermodynamic textbooks have
tables showing the latent heat of vaporization for different conditions.