In the newest Sid Meier's Pirate! Treasure Ships come in the form of galleons, or indianmen.
Now Some ships have the words TREASURE SHIP above the actual classification of ship. Unfortunately I don't have an exact example but here is a reference.
The image is that of a Ship of The Line. As you can see the words "New Warship" are listed above the actual classification of "English Ship of the Line".
So, the question is, what is the cash difference between a Treasure Galleon and a Treasure Ship?
What are the different goods that they carry?
Are the escorts different per ship?
(Picture from hookedonpirates.com)