"That's the going rate today, Mr. Farwell. It may change tomorrow; I haven't checked the market." This is a beautiful little piece of double irony, intended ironically by the speaker but also unwittingly reflecting the real problem he doesn't know about. By itself, it makes the whole third act worthwhile. Even if you've already figured out what's coming, as I had, you don't expect one of the characters to pronounce his own sentence.
The only thing I would change would be Farwell's murder of De Cruz, because it partly spares De Cruz the consequences of his own actions. Every one of those gold bars weighs a good fifteen or twenty pounds. By extorting gold from Farwell, De Cruz is adding to his own burden and reducing his chance of survival. Leaving Farwell to die of thirst when he runs out of gold to buy water with, only to have De Cruz collapse because he's carrying twice his share and has sold half his water - now that would have been a true character-is-destiny moment.