Commodity Pricing With Sliding Scale Discounts

To all pricing gurus.

Here's the scenario...

Need price and apply discounts to a commodity. 

Using a price list to store the monthly average index price of the commodity.  This means that the list price for the commodity will change every month, the prior month's price will be end dated so that there will only be one active price at any time.

On some customers I will need to deduct freight from the index price before I apply volume discounts.  The volume discounts are a sliding scale based on the net market price (index price net of freight), the discount will decrease as the market index price increases.