Relationship between flow and price in wheat

Negative relationship between surplus/deficit

In my last post I presented a map showing counties which were in surplus or deficit. I am convinced there is a relationship between price and surplus or deficit, with the price difference between counties being due to transport costs. I regressed (there you are again!) the flow against the wheat price, and found a strong statistical relationship (p=0.007). Here is a scatterplot which admittedly looks---well, scattered! But you can tell by eye that the relationship is negative. The greater the deficit, shown by having a negative sign on the y axis, the more you pay for wheat, price of wheat on the x axis. Now I need to try somehow to build a mathematical relationship between flow, distances travelled, and wheat prices. If I can find this relationship, then I can set it to zero on one side of the equation to find the 'limit of cultivation'. If that limit happens to be the border between Devon and Somerset----problem solved and game over!