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

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

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