When you make a decision means that you take an action course and leave othe free. You take advantage of some oportunities but "lose" other oportunities. Those opportunities that you let go are the opportunity cost in which you incurr any time that you choose. Your economical analysis (and probably in all life dimensions) must include the opportunity costs to make a decision that leaves you better than you would be if you had taken a different decision, this is your expected benefit should overcome the opportunity cost.