A ticket to an Eric Clapton concert costs $45. If you have a ticket, you can "scalp" it (sell it illegally) for $75. To a ticket holder, the opportunity cost of actually attending the concert is

You could easily still go to the concert but you would risk getting caught

Opportunity cost is what you give up to do something

if you go to the concert, you spent $45 dollars but lose the opportunity to sell the ticket

if you sell the ticket illegally, you get $75 at the cost of not seeing the concert

the opportunity cost of attending the concert=75+45=$120

the opportunity cost is 120 dollars