There are two prices possibly involved in calculating this cost. You take the price at the beginning of the offer period and compare it to the price u actually paid. You use the lower of those two numbers. From that you subtract the discount in the plan, usually something like 15% of whichever of the twp prices was lower. Your company usually provides a sheet showing all these amounts for you. Maybe it's still on their website if you don't have the paper. It's confusing to do this without that data from the company and pretty simple if you have that information.
