How do I create a formula that would alow me to select a value in one cell
and subtract it from a value I enter in another cell.

IE a product with 3 versions cost me 1.00, 2.00 or 3.00 depending on the
version

I want to create a forumla that alows me to select one of the build costs
then enter a selling price in another cell and the 3rd cell would subtract
the value of the build cost from the sales price and give me my margin.