Demand level is determined by multiple factors:
1) population of the station, roughly proportional to the square root of the population
2) the rarity of the good - Water has much higher tonnage than Nerve Agents, for example
3) a random skew for the station, so that not every 1M population refinery has an identical market (I call this "specialisation" - you can see some figures for it at
https://cdb.sotl.org.uk/specialisation )
These first three between them determine the "baseline demand cap". To take a specific example, for Concordia Hub in Edge Fraternity Landing, this is 9785t. If the BGS state is "none" and no trading takes place, the demand level will eventually return to this number.
4) the BGS states in effect. These multiply up the baseline demand cap to get the "current demand cap" - data tables here:
https://cdb.sotl.org.uk/effects
5) the amount of trading that takes place - every time someone delivers a tonne [1], the demand reduces by 1t, of course, but then it regenerates back to the cap at a rate determined by the station and the commodity (in general, the more expensive the commodity, the slower it regenerates) and possibly also affected by the BGS states
This combination gives the current tonnage demand for the station.
Price level is then determined as follows:
1) a baseline price for the good
2) again, multiplied by the effects of the BGS states
3) as demand falls below the cap, this price falls too, bottoming out at about 25% of the cap (again, varying with commodity). How much the price falls depends heavily on the good - the expensive gems can lose up to 50%, whereas some other less elastic goods only lose about 10% of their price.
4) then you finally get "bulk sales tax" applied if demand is >25% of the cap and your hold contents are a significant percentage of current demand
Here's a graph containing examples of most of this - on the left side, you've got prices and demands varying as goods as sold and demand regenerates over time, then on the right side you have a big "step" jump up and down as the faction adds Expansion to its state list, then removes it again.
...and if we zoom in on the 9th
...you can see the price flattening out on the way down well before the demand does.
What you can also see from the graphs is something else significant - for most commodities there just isn't anywhere near enough routine trade volume for demand to ever be below the cap, so states and to an extent bulk tax are basically the only determinants of price. Platinum and some other mined goods are an interesting exception to this and bring the full complexity of the economic BGS into play.
However, what about that system that has a demand of 11 items, and willing to pay a high premium for them?
If you rock up with any more than 3(?)T of platinum in this case, aren’t they just going to drop that premium offer back to galactic average?
With a demand of 11 items, the chances are for Platinum that the system is already basically at 0% of its demand cap, so the price will be in the (local) minimum anyway, and no bulk tax will apply.
If the demand
cap was 11T for a system, then yes, showing up with 3T of Platinum would be significant and drop the price.
How do you tell what the demand cap is? You can't from EDDB/Inara. Make a guess from the system population, and the state effect and specialisation tables, of roughly what the demand cap might be.
[1] The BGS states are themselves obviously also caused by player activity in the "political BGS" rather than in the "economic BGS", though deliveries of cargo to meet demand in the "economic BGS" will also be trade transactions in the "political BGS" - the interactions are individually all very tangled though the aggregate effect is often a bit simpler to understand.