Step 1: Do this stuff.
Step 2: Profit!
In EUIII In Nomine and Heir to the Throne, trade good prices are determined by a series of modifiers that affect Supply and Demand.
Supply is calculated individually for each province that produces the good.
Demand is calculated individually for all provinces in the world.
When supply > demand, prices fall. Vice versa. Simple.
I'm not going to list all of the effects, but in general, looting, occupation, and blockades reduce supply significantly. Significant ones will be bolded.
Grain:
Supply is affected by: Free Subjects 1+ (-50%), Free Trade 1+ (-80%), a unit standing in the province (-90%).
Demand is affected by: Stability 2+ (+10%), war (+10%).
One of many trade goods where being Free Trade is beneficial. If you control most of the grain in the world, parking 1 infantry in each grain province will make you rich.
Wine:
Demand: 2+ RR (+10%), Refinery (+100%), province religion = muslim (0 demand), Stability 2+ (+10%)
Going Muslim is bad for winesellers.
Wool:
Supply: Aristocracy 2+ (+50%)
Demand: Shipyards and Textile Manus (+20%), cloth in province (+100%), being overseas (-20%), and Trade is less than 10% total income (-50%)
Cloth:
Supply: Plutocracy 1+ (+50%)
Demand: Shipyard, Textile Manu, and Regimental Camp (+20%), looting (-90%), and Trade is less than 10% total income (-50%)
Nothin' real major for either of these.
Fish:
Supply: Land 1+ (-50%)
Demand: Catholic province (+20%), owning no ports (-90%)
Being Catholic helps here. So does making sure there are no portless major powers. Ironically, high land makes fish more valuable.
Fur:
Demand: Workshop (+10%), war (-80%), Revolutionary Republic govt (-90%), Quality 4+ (+200%)
Apparently, this is mocking France invading Russia in the winter. High quality is really the main thing here, since you should be building workshops anyway.
Salt:
Demand: Stab 2+, Regimental Camp, Fish (+10%), Serfdom 1+ (-50%).
Serfs just don't really use much salt.
Naval Supplies:
Demand: Naval Manu or Shipyard (+10%), Land 1+ (-90%), Admiralty (+400%), capital when you have 20 big ships and are at war (+1900%), capital when you have 50 big ships and are at war (+900%)
Being on the naval end of the slider is the most important. The big percentages look really big, until you realize they only apply to your capital.
Copper:
Supply: Weapons manu (+100%), Free Subjects 1+ (+10%)
Demand: Not Eastern/Western (-90%), own artillery (+10%), Land Tech 30 (+10%), peace (-50%), Regimental Camp (+5%), Weapons Manu (+20%)
If you own Iron provinces, do not build weapons manus in your copper provinces, or you'll increase supply and thus reduce price. If you have a couple copper provinces, as soon as you can build artillery, consider building one, even if you aren't going to use it. Finally, wipe out those Muslim/Indian/Chinese/African/New World nations. It's for the economy.
Iron:
Demand: Not Eastern/Western (-90%), own 20 infantry(+10%), Land Tech 30 (+10%), peace (-50%), Regimental Camp (+5%), Weapons Manu (+20%), Land 1+ (+10 %)
Wipe out those Muslim/Indian/Chinese/African/New World nations. Also, get 20 infantry (which you'll probably do anyway in HTTT, but maybe not in IN).
Slaves:
Supply: Abolish Slavery Act (0%)
Demand: Cotton, Tobacco, Sugar, Coffee provinces (+900%), everything else (0%), Abolish Slavery Act (0%), capital is overseas from slave-using provinces (+100%)
Essentially, Slaves are worthless until the New World is heavily colonized. Even then, with a base price of 5, almost everything else in the game is better. Maybe if you own 1 slave province, every slave-using province, and everyone else has abolished slavery and wiped out the rest of the slave-producing provinces, but that's kinda far-fetched. Thus, pass the Abolish Slavery act ASAP so your slave provinces will produce something better.
Ivory, Tea, Chinaware, Spices, Coffee, Cotton, Sugar, Tobacco
Demand: No Marketplace (-80%), Customs House (+50%), Constable (+50%), Tax Assessor (+50%), Mercantilism 2+ (-50%), Gold income % >10% (+10%), War (-10%), 2+ Embargos (-10%), RR 2+ (-50%), 20+ Prestige (+10%), Textile Manu (+200%, cotton only), Refinery (+200%, sugar only)
Colonial goods are nearly worthless before advanced nations drop marketplaces like crazy, and really take off once everyone starts putting down the rest of the buildings. Mercantilism nerfs colonial goods.
Step 2: Profit!
In EUIII In Nomine and Heir to the Throne, trade good prices are determined by a series of modifiers that affect Supply and Demand.
Supply is calculated individually for each province that produces the good.
Demand is calculated individually for all provinces in the world.
When supply > demand, prices fall. Vice versa. Simple.
I'm not going to list all of the effects, but in general, looting, occupation, and blockades reduce supply significantly. Significant ones will be bolded.
Grain:
Supply is affected by: Free Subjects 1+ (-50%), Free Trade 1+ (-80%), a unit standing in the province (-90%).
Demand is affected by: Stability 2+ (+10%), war (+10%).
One of many trade goods where being Free Trade is beneficial. If you control most of the grain in the world, parking 1 infantry in each grain province will make you rich.
Wine:
Demand: 2+ RR (+10%), Refinery (+100%), province religion = muslim (0 demand), Stability 2+ (+10%)
Going Muslim is bad for winesellers.
Wool:
Supply: Aristocracy 2+ (+50%)
Demand: Shipyards and Textile Manus (+20%), cloth in province (+100%), being overseas (-20%), and Trade is less than 10% total income (-50%)
Cloth:
Supply: Plutocracy 1+ (+50%)
Demand: Shipyard, Textile Manu, and Regimental Camp (+20%), looting (-90%), and Trade is less than 10% total income (-50%)
Nothin' real major for either of these.
Fish:
Supply: Land 1+ (-50%)
Demand: Catholic province (+20%), owning no ports (-90%)
Being Catholic helps here. So does making sure there are no portless major powers. Ironically, high land makes fish more valuable.
Fur:
Demand: Workshop (+10%), war (-80%), Revolutionary Republic govt (-90%), Quality 4+ (+200%)
Apparently, this is mocking France invading Russia in the winter. High quality is really the main thing here, since you should be building workshops anyway.
Salt:
Demand: Stab 2+, Regimental Camp, Fish (+10%), Serfdom 1+ (-50%).
Serfs just don't really use much salt.
Naval Supplies:
Demand: Naval Manu or Shipyard (+10%), Land 1+ (-90%), Admiralty (+400%), capital when you have 20 big ships and are at war (+1900%), capital when you have 50 big ships and are at war (+900%)
Being on the naval end of the slider is the most important. The big percentages look really big, until you realize they only apply to your capital.
Copper:
Supply: Weapons manu (+100%), Free Subjects 1+ (+10%)
Demand: Not Eastern/Western (-90%), own artillery (+10%), Land Tech 30 (+10%), peace (-50%), Regimental Camp (+5%), Weapons Manu (+20%)
If you own Iron provinces, do not build weapons manus in your copper provinces, or you'll increase supply and thus reduce price. If you have a couple copper provinces, as soon as you can build artillery, consider building one, even if you aren't going to use it. Finally, wipe out those Muslim/Indian/Chinese/African/New World nations. It's for the economy.
Iron:
Demand: Not Eastern/Western (-90%), own 20 infantry(+10%), Land Tech 30 (+10%), peace (-50%), Regimental Camp (+5%), Weapons Manu (+20%), Land 1+ (+10 %)
Wipe out those Muslim/Indian/Chinese/African/New World nations. Also, get 20 infantry (which you'll probably do anyway in HTTT, but maybe not in IN).
Slaves:
Supply: Abolish Slavery Act (0%)
Demand: Cotton, Tobacco, Sugar, Coffee provinces (+900%), everything else (0%), Abolish Slavery Act (0%), capital is overseas from slave-using provinces (+100%)
Essentially, Slaves are worthless until the New World is heavily colonized. Even then, with a base price of 5, almost everything else in the game is better. Maybe if you own 1 slave province, every slave-using province, and everyone else has abolished slavery and wiped out the rest of the slave-producing provinces, but that's kinda far-fetched. Thus, pass the Abolish Slavery act ASAP so your slave provinces will produce something better.
Ivory, Tea, Chinaware, Spices, Coffee, Cotton, Sugar, Tobacco
Demand: No Marketplace (-80%), Customs House (+50%), Constable (+50%), Tax Assessor (+50%), Mercantilism 2+ (-50%), Gold income % >10% (+10%), War (-10%), 2+ Embargos (-10%), RR 2+ (-50%), 20+ Prestige (+10%), Textile Manu (+200%, cotton only), Refinery (+200%, sugar only)
Colonial goods are nearly worthless before advanced nations drop marketplaces like crazy, and really take off once everyone starts putting down the rest of the buildings. Mercantilism nerfs colonial goods.