In a combinatorial exchange, rules can be input for processing bids and a desired exchange objective can be defined. A subset of the rules can be used to determine 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, new allocations can be determined using different subsets of rules each time until the desired exchange objective is satisfied. At least one of the bids can include a reference ratio; a discount; a price associated with a quantity Q1 of a first item; a price associated with a quantity Q2 of a second item; and a rule that causes said discount to be applied to an average price of Q1 and an average price of Q2 for each instance of the allocated quantity Q1 over the allocated quantity Q2 that equals the reference ratio.