This paper aims at putting forward the analytical stake of the few passages that Thomas Aquinas devotes to prices and exchange, mainly in the Summa Theologiae. At first sight, his objective is to enlighten a confessor vis-à-vis his penitent, or the judge in an ecclesiastical tribunal, by way of a group of normative prescriptions, tending to distinguish that which is just in commercial transactions from what is not. But on second thoughts, this objective leads the author to a more complex construction, which involves establishing a referential norm - the just price - to which the transaction price should be compared. It is recalled here that resorting to the just price - the discussion of which chiefly takes place in the commentaries on the Ethics avoids any consideration of individual behaviour. However, this last comes to the forefront when the issue dealt with is to explain the reasons why such a transaction price is equal to, or on the contrary departs from the just price. Thomas Aquinas' treatment of this issue allows one to acknowledge a) that individual behaviour is characterized by virtue or by vice in various informational contexts, and b) that the making of a transaction price is the result of a negotiation process between buyer and seller. In a context of correct information, where the partners are both virtuous, Thomas Aquinas explains why the transaction price is equal to the just price - in the exchange in se - or could differ from it - in the exchange per accidens. But focussing on the exchange in se, both an asymetry of information and the vice of at least one of the partners give rise to deception strategies leading to transaction prices, presented as just by the party who knows it is not, and agreed upon as just by the deceived party. Lastly, the possibility of retaining information during the negotiation process paves the way for the opportunity for the virtuous seller to protect himself against the higher power of negotiation of a possible vicious partner. Although aiming at a different goal, Thomas Aquinas thus provides a complete theory, not only of the just price, but more generally of exchange, in which ethical considerations become decisive in determining transaction prices.