Cost of fuel scales up with ship cost, as you undoubtedly know. But fuel usage per LY also scales up as you use larger percentages of the ship's jump capacity. That is jumping 10 LY at once uses more fuel than jumping 5 LY twice - quite a bit more. Higher rated FSD also uses less fuel for a given jump distance, so an otherwise equal ship with a D-rated FSD will use more fuel jumping 10 LY than the same ship with an A-rated FSD. Even ship mass matters.
It's a very complicated equation.
This was discussed in another thread yesterday (can't find it right now), but basically the fuel usage is approximately logarithmic, i.e. jumping twice the distance in
one jump uses four times the fuel than doing it in two equal sized jumps assuming everything else is equal (FSD type, fuel weight, cargo weight etc.) That's why the eco routes in densely populated space can get you over 180Ly on half a tank in a Cobra, but the fastest routes will have you out of fuel in 80Ly. It's costly to do even 30Ly in a Cobra on fastest vs eco. 2000Cr instead of 500Cr.
Putting in a big FSD and doing eco jumps is by far the cheapest way to travel, but it's sooooooooo boring jumping 3-5Ly in a ship that can do 18 or 20!