I'd like see a new link under the "Items and Inventory" tab, in the "Pricing and Costs" section.  The link/button would be called "Special Customer Pricing".

In that screen, there could be a customer lookup in a header, then  line items that include the item lookup, which triggers a default price in a text box that comes from standard pricing. The default could be overwritten by the user with the special price.  I would also include a text box for notes so users could make any special notes regarding the pricing.