A Listbox, like a Combobox, is intended to be used as a selection device, not as a primary data display device. As such, it doesn't have the functions available to it to do this kind of thing, as a Continuous or Datasheet Form would have. To do this, I think you'd have to do it in the query that you're using as the Row Source for your Listbox.
But I'm curious as to why you have to calculate the price for an item. In the normal scheme of things, An Item would have A Price, which would be displayed in the Listbox. Do items have more than one price? What kind of calculation do you need to do?