Answer:
Cost of 4 days time: In terms of earnings sacrifised from those working days, or any qualitative value of that time.
Explanation:
Opportunity Cost is the cost of next best alternative sacrifised while choosing an alternative.
It might be material, immaterial .
Eg 1 has material (monetary) opportunity cost i.e salary. Eg 2 has immaterial opportunity cost i.e satisfaction.
Admission ticket for Bonnarro would have had opportunity cost = its price i.e $554.50 as the amount sacrifised to get that. However: as given the ticket is gifted free ; the opportunity cost of that is 4 days time sacrifised to attend it. The value of that sacrifised time may be in form of :- earnings lost from those 4 working days sacrifised (material) ; or 4 days time to be spent with family sacrifised (immaterial).