You have tickets to go to Mexico (Cancun specifically) over spring break. Just this week your best friend informs you that s/he is getting married over spring break and would like you to be in the wedding as an attendant. Which of the following is a sunk cost that should not be relevant to your decision as to whether be in the wedding or go on the trip to Mexico?

1. The cost of the airline tickets to Mexico
2. The amount of refund you could get on the airline tickets to Mexico
3. The cost of the clothing you will have to buy/rent to be in the wedding.
4. The fact that you have never been anywhere for Spring Break and were really looking forward to going to Mexico