I have a bad habit of misreading questions at times, but if I've read it correctly, you have a fixed cost, a combo box with quanity, and want to add the result to a listbox?
VB 6 is all I have, so I dunno about other versions. In the combo's click event, you might try something like
List1.addItem Format(CStr(cost_of_item * CSng(Combo1.List(Combo1.ListIndex))), "####.#0")
Try that and see if it does what you need.