The Roman law of sale initially had no requirements in respect of the price paid for a thing bought. As long as the price agreed upon was the basis of consensus between the purchaser and the vendor, there was nothing that precluded it from being either exorbitant or too little, and there was no remedy if you paid too much or received too little. The idea behind this was that it was the prerogative of everyone to conclude a foolish bargain if he wished ("he" because only men concluded contracts in earlier times). For this reason the adage caveat emptor (=buyer, beware) developed. It was the obligation on the buyer to ensure that he was not lead up the garden path.
Later the concept developed that the price agreed upon had to be iustum or verum, that is "just" or "true" in the sense that it had to correspond generally with market value for that particular kind of thing. During the post classical era, the notion of laesio enormis (literally "enormous loss" or "damage") developed in respect of sales of land. This entailed that the vendor who sold land for less than half the real or market value, could resile from the agreement and claim return of the land, obviously upon repayment of the purchase price, unless the purchaser was prepared to make up the shortfall to pay the real or full value. The main reason behind this was that the Roman empire was experiencing a large degree of agrarian depopulation, and the rich were buying up the land belonging to the poor for next to nothing. The remedy made available in terms of the doctrine of laesio enormis was aimed at stabilising the price of rural land on the one hand, but also to prevent an agrarian crisis with insufficient small farmers to provide in the needs of a vastly hungry empire.
A further possible reason was the fact that the empire was in a state of continual war, and the requirement was that serving soldiers had to provide their own kit. The various classes, depending upon their financial means, were required to kit themselves out in accordance with what they could afford. If you owned land, more was required than if you did not, since the landless generally were exempt from military service, and this obviously gave rise to dissatisfaction among the poorer landed classes who could often not afford both taxes, and the expense of maintaining the military kit. It was therefore an inviting prospect to simply sell your land to the first purchaser interested, and to live on the dole, thereby being exempt from military duty.
Needless to say, the remedy was crude, in so far as it did not address the reverse problem, where the seller receives too much for land that later proves to be valueless.
The Roman solution has not, to the best of my knowledge, been received into modern jurisdictions. In the United Kingdom, the doctrine of consideration applies, where the parties are required to agree on a quid pro quo in any contract of sale, regardless, however, of whether the performance is in any way related to the value of the object or not. So long as there is consideration given, one can validly contract the sale. In South Africa, the doctrine of consideration per se is not followed, and it is everyone's prerogative to make a fool of him- or herself, although our courts have stated that the law will not allow another to make a fool of you, regardless of what you may wish to do yourself.