IMHO Pricesmart seems to apply the same percentages on everything that they do in the states, figuring a little extra (probably the actual amount) for transportation cost and duties. I doubt that they have any buddy deals with corrupt officials for cheating on import taxes but their prices are pretty fair on most things none the less. Foreign food is probably the best buy in the store since they don't mark it up nearly as much as the other grocery stores in town.
Plaza Lama is relatively new (at least in Santiago, I don't know about the capitol) and are trying real hard to take some market share away from Distribuidoras Corripio and other more established electrodomestic distributors, so they are selling many items at or near cost, this is hard for anybody to compete with. The only problem is they handle alot of factory seconds and refurbished products, and won't usually tell you that at the time of purchase.