Create a New Value for Items?

My goal is to create a currency mod based on what the actual value of items would be in real life. Right now I have a bunch of money items and a block for exchanging items for money, but I want to create a new value that is attached to each item called "Worth" (or something similar), that will determine how much money you get from the item, ie. an iron ingot would have a Worth value of 81.11, approximately the value of 1/9 of a cubic meter of iron. Any help is apreciated!