I assume it's specified for each individual fuel station in the level editor, and packaged into each level.
There doesn't appear to be any global xml file with any reference to hardmode yet.
I'm really hoping that if they don't create a way to customize hardmode in the game itself, that they at least expose some of these values in a new xml file somewhere - just some global multipliers, so we can tell repair points to cost 10x more, fuel to cost 0.8x as much, recovery to cost 4x more, etc, etc.
No, that's my point, there's nothing currently in the initial.pak. Individual maps appear to be packaged up into a single file, and I presume each of those specifies the fuel cost at each gas station.
I haven't used the level editor, but I suspect currently if you wanted to change the prices, you'd need to open up each of the base-game maps in the editor and change the values there somehow, and then repackage them. I don't know if you can even do that with the base-game maps...