How can we see on the Master table, the cheapest company from the Details Table?

I'm going to assume you have 3 tables in this set up.

(1) Products
(2) Vendors

and the joined table of call it (3) Product Sources

Each vendor & each product will have many product sources.

Then you can enter the prices for each supplier on the items.  Then you can summarize the cheapest supplier cost.  And really do anything else you need to.

If you need more details let me know.