The value depends entirely on whether the two parties both agree whether assembly of the figures increased or decreased the value.
Scenario One: Person A sits down and puts together a box of models. Person B walks over, likes how the models were put together and is glad to avoid the hassle of putting the box together, so offers a trade of the same price.
Scenario Two: Person A sits down and puts together a box of models. Person A tries to sell the box to Person B, who thinks about all of the weapon choices and options that they can't use out of the box now. So the box of figures is worth *less* to Person B than the price that Person A paid for it.
You cannot establish an equitable trade if one person is in Scenario One and the other is in Scenario Two.