Vendor Price vs. Price on Bill Report

I have been working on a saved search based on vendor bills that will allow me to compare the vendor price (as entered on the vendor tab of the item in question) with the price on the invoice (bill) from the vendor.

This would allow very quick auditing of the billed price against the contracted price as entered by vendor on each item for overcharges by the vendor.

I am finding that the only field that I can source is the purchase price from the Purchasing/Inventory tab on the item. The formulas for this method are as follows:

Overcharge Ea = ({amount}/{quantity})-{item.cost}