This number is the maximum rate at which food can be converted into gold for that region, when the gold is the max gold production and food the max food consumption. Therefore, if the price for food goes over that value, then it is a net loss in value to purchase food. The solution is not to purchase any food and let population decline until purchasing food is profitable.
When pop goes down due to starvation, it usually takes a while to recover. So you still have to factor in the gold you might have earned at max production, during the recovery period, to offset the gold you might spend on the food. I'm too lazy to do the math, but I'd guess the limit is still a little above 50 gold.
