Kid Charlemagne said:This happens all the time today. Farmers have a bumper crop, and suddenly what they were selling for $25 a bushel now only sells for $12. Their crops are growing better than ever, but they're making less money, because everyone's crops are growing more. This is why the government pays farmers to NOT plant crops.
Sure, but this happens in modern times because our ability to produce food far exceeds the demand in first world nations, and in poorer (and hungrier) areas there isn't enough wealth to make it profitable to sell food to them. Quasi-medieval economies are unlikely to have those characteristics, even in the comparitively small area in which food can be transported before it spoils. More fully magical economies might, though.