Cheap, yea. But also extremely dirty. What is the mpg ona tanker using crude diesel again? Cool, now swing that tanker three or four times through Singapore.
Okay, but you have to divide by the entire cargo of the ship, which is quite enormous. So I still don't think it's something easy to estimate without doing the math.
The bunker fuel used by cargo ships is a byproduct of the refining process after they extract the higher quality fuels.
The fuel has to go somewhere. We could bury it back in the ground because we don't want to burn it, but we need some fuel to power international trade.
In terms of energy per ton mile, you can't really beat a cargo ship.