So "supply" isn't really supply, then? Confusing.
"Supply" is all the units of a trade good produced in the world, with the province-by-province modifiers in effect. So supply is a modified "supply".
Demand is where things are a bit odd I think.
Here's a sort of optimization scheme for price manipulation:
Things to do:
Stability = 3 (1.1 Salt/Grain/Wine Demand)
Serfdom/FreeSubjects >= 1 (1.1 for Copper Supply, 0.5 for Grain Supply)
Mercantilism/FreeTrade >= 1 (0.2 for Grain Supply)
Quality/Quantity < -3 (3.0 for Fur Demand)
Gold Income % >= 10% (1.1 for non-European Good Demand)
Keep 1 artillery unit in each province (0.1 for Grain Supply and 1.1 for Copper Demand)
Build all buildings, especially regimental camps, marketplaces and refineries (lots of bonuses)
No embargoes (eliminates penalty for non-European good Demand)
Keep revolt risk at 0 (eliminates severe penalty for non-European good Demand)
Trade Income % >= 10% (eliminates penalty on Wool and Cloth Demand)
Be at peace (eliminates penalty on Fur Demand while at war)
*if you like Catholicism, 1.2 to Fish Demand
*let yourself be looted/blockaded/conquered for modest bonuses if you like
*Choose your poison for Aristocracy/Plutocracy -- either Wool/Cloth Supply will be harmed
*consider Admiralty for a 5.0 to Naval Supplies Demand
[EDIT: What is clear is that the penalties associated with going Mercantilistic are significantly negative, and should have a big impact on the "Mercantilism v. Free Trade" debate.]