Commodity pricing volatility is unrealistic. The price of a commodity should not be able to drop from 100 to 1 from a few sales even if they are lots of a hundred. Many hidden forces work to stabilize prices.
My view of an accurate pricing model for this game would have very stable base prices for each commodity along with the more dynamic spot prices that are affected by buy and sell pressure. The base prices would change gradually based on long term trends in supply and demand and probably some built in inflation. The spot price is based on the base price and has a powerful force tugging it back to the base price, like gravity. Buying or selling temporarily affects the price like stretching a rubber band. Initially, the rubber band stretches easily with minimal force, but as it gets more taut (spot price variance from base price) it resists being stretched and much greater force is required to move it a much shorter distance until it refuses to move any further. With this model the spot price would drop from 100 down to 90 rather easily, another large sale would only drop it to 85, and so on. The greater the percentage variance from the base price the less it would be affected by additional sales (the same for purchases with rising prices). Minimal forces are required to move towards the base prices, large forces are required to move away from base prices.
In the real world corporations don't rely on spot market prices to purchase supplies or sell their product. They enter into long term contracts with fixed prices for supplies and typically only sell 25% of production on the spot market. I'm not suggesting you build long term contracts into the game but the rubber band model is one method to more closely simulate real world economics.