Complex sentence: A choice needs to be made between several mutually exclusive alternatives; assuming the best choice is made, it is the "cost" incurred by not enjoying the "benefit" that would have been had by taking the second best available choice.
Split into simpler sentences: A choice needs to be made between several mutually exclusive alternatives. Assuming the best choice is made, it is the "cost" incurred by not enjoying the "benefit" that would have been had by taking the second best available choice.