[gus_massa]

My guess is by weight. (It also may be by volume, that I guess is very important to transport illegal things, but it's too abstrtact. [1])

Comparison by weight is useful to give an idea of the price. It's just like measuring distances in football fields. Don't worry too much about that. Also, a cargo ship full of gold [2] is probably very valuable, but a magical cargo ship full of rhino horns will probably collapse the market and be worthless.

Here is a list of most expensive materials per weight https://brightside.me/articles/the-17-most-expensive-materia... it includes saffron and antimater that have a very short shelf life, a few radioactive things that are expensive only beacuse they are difficult to produce [3]. If I had to stock huge quantities, I'd be very conservative and store gold and platinium.

[1] Gold has a huge value per volume ratio. Toilete paper not.

[2] Assuming it doesn't sink.

[3] Aluminium used to be very expensive, until someone invented a method to make metalic aluminium easily.