Question on a salesforce case study

Case: I run an organization that sells 3 products, blue, red, and green gift boxes. We are targeting stores in shopping malls to use our gift boxes for gift wrapping their merchandise. Our sales people call and visit stores at the malls talking to store owners about our boxes. To help the sales efforts, our sales people carry sample boxes that they distribute to store owners that show some interest.
Reps need to be able to track their customer base, sales made and since left samples are at a cost they need to track how many samples each sales person is giving out and the associated cost. Create a basic system that will support these 3 needs.
These are only high level details, so feel free to make assumptions or ask questions.
Do not use the standard salesforce.com Opportunity object.

So far, I’m thinking to create a custom object with master-detail relationship with Account, and lookup relationship with Product. Does this sound right? And from here, what would be my next step? How can I track the associated cost? (will I need a formula?)
I appreciate any help. Thank you so much!!