If I'm a merchant, I'm going to lie about how much 'prescious cargo' is worth. Every good that a merchant touches has at least two "prices", the buy price and the sell price. So, the money should be based on:
1) Something easily observed, like ship size and whether the ship is "heavy" or not.
2) Something objective, like the number of passengers or volume / weight of the cargo.
3) Sure, this penalizes people shipping tons of cheap grain. But who the heck would bother to ship tons of cheap grain? Don't worry about harming the market, it will adapt. Do worry about rules that are hard to police, such as those which require that you know the sale price of stuff.
-- N