You want to only sell goods in fiefs with high base prices and buy in fiefs with low base prices. Click on villages on the map screen and it'll show price and current S&D. Look for fiefs with prices over 20 when planning your route.
Once inside the villages, be sure to check the current price on the info screen too. If the S&D is too low, the price of the goods is lower as well and it'll say for example 17/24 if 24 would be the normal price it's now 17. On the village info screen it shows the prosperity (S&D) and what the maximum amount it will recharge to per day. Once that recharges to full, so will the price.
You can calculate how much you'll earn per good by taking price x distance bonus + price - tax = profit. So for a village with a 24 price... .24 x 350% bonus = 84 + 24 = 108 - tax = profit.