Assigning or setting a string equal to a specific number

Basically, I have a list box with different items that can be selected.  Each item is alloted a particular price.  Each time a item is "purchased" I need to tally the quantity and add up the total (price).  In this situation, how do I assign a numerical value (the price) to each item in the list box so that I can have the program tally and add up the total?