When a BA considers opportunity costs, they are performing a cost-benefit analysis to determine the best solution. This falls within the task of "Analyze Potential Value and Recommend Solution", where different design options or requirement implementations are evaluated for their expected value and opportunity costs (what must be forgone when choosing one option over another).
This involves:
Comparing expected benefits vs. associated costs
Evaluating risks and missed opportunities
Recommending the solution with optimal value delivery
BABOK Reference:
Found in the Requirements Analysis and Design Definition knowledge area, this task emphasizes assessing the relative value of each solution option.