LEV965 Ona'at mammon (price deception or price fraud) is a very important prohibition that teaches us that we have to "play fair." The concept is related to theft, although unlike geneivat da'at (deception), ona'at mammon is a specific provision found in the Torah [this verse]. The Mishnah articulates very specific guidelines as to what constitutes price fraud: selling an object for more than 1/6 above what it is "really" worth. Interestingly--and very much at odds with our normally accepted ideas about fairness in pricing--it is equally forbidden to underpay. Paying more than 1/6 below the real price is also forbidden, and such a transaction can be legally nullified. A reasonable profit is allowed, and some variation in pricing is acceptable, but overall prices should be "fair." One of the biggest problems in applying this principle is determining what constitutes the "same product." Two stores may be selling what appears to be the same product, but one store might provide a better warranty, or have different payment terms, or offer much better customer service, and so on, all of which could be sufficient to say that the product is not exactly the same and a price differential greater than 1/6 is justified. (By Barry J. Leff, "Jewish Business Ethics")