Formula to Calculate Margin in Excel when Retail Price has a multple qty (ex: 2/$4)

I'm looking for an Excel formula that will calculate a margin based on a multiple qty retail like 2/$4.00, 4/$5.00, 10/$10.00, etc.

Column A has the retail price. Column B has the unit cost.

The formula I usually use is =(A1-B1)/A1.

TIA!