Since the cost can correctly only be evaluated when the transaction (cycle) has taken place, take a look at that for cost calculation of previous trades, and use some algo to predict the same for future scenario which you are supposed to be dealing of.

My advice on approach would be that you should consider it as a big auction house with many items, if you remove the items you're still left with the house, what's the value of the house ? There can be different approaches, yet there can't be a repetitive answer.