In a combinatorial exchange, a set of rules can be input for processing of bids received in connection with the exchange. At least one bid can be received from each of a plurality of exchange participants and a desired exchange objective can be defined. A determination can be made as a function of a subset of the rules if an allocation of the bids exists that is optimal for the type of exchange being conducted. If the desired exchange objective is not satisfied, the step of determining an allocation is repeated utilizing a different subset of rules each time until the desired exchange objective is satisfied. It can be determined if the exchange is overconstrained and, if so, rules can be relaxed. Also or alternatively, the demand for a quantity of an item can be increased or decreased based on the average cost of the item.