Prices are better from a farm because you’re skipping two steps on the distribution chain, at least - a food warehouse and the grocery store. Could be three, some grocery stores buy from an intermediate warehouse distributor that services smaller stores.
So potatoes might be sold at .20 on a farm and .50 at the store, because they need to be sold twice to reach the store, transported twice, bagged, washed, stored twice, and finally placed in the retail front for sale.