Formula/Algebra help?

Sorry. I thought I may have posted it in the wrong forum (as there were no responses, and I assumed this would be an easy one for you excel gurus).

As for a homework assignment... I wrote it out in the best way I could to be clear on what kind of formula or function I was looking for that could help me learn how excel could best accomplish my goal. I'm building a material list (for our salesmen) that auto calculates the cost of products per vendor and auto-selects the most cost effective vendor based on the quantities necessary input by the salesman.