As for the supply/demand thing, it does affect the price, but I don't think it's the only thing that does. In simple terms, a low supply means that the product is rarer, which means more expensive to purchase, high supply means there's more, so it's cheaper to purchase. Conversely, low demand means they have a lot of it, and will pay less when buying more, high demand means they have very little, and will pay more to refill their stocks. Even if it was the only thing that affected price you still can't translate say Medium Supply into a fixed value, there are nuances to it, which is why price has to be entered manually. But you don't need to enter the price really. The point of the Low-High Supply-Demand structure is to be able to approximate the best trade routes, which is how the trade window works currently. For example the best possible result you'll get, not knowing the prices, will always be High Supply (cheapest buying price) to High Demand (Highest selling price). The prices can be entered optionally to show the potential profit (per ton, currently) for each trade route.
I hope that answers your questions about that!
