Because they weren’t ready to steal it again just yet
The opportunity cost in this scenario is Mikael's decision to forgo seeing Ouro Preto during his stay in Brazil.
Opportunity cost refers to the decision making process people use in terms of how they spend their time, resources, or money. This term refers to the loss that a person suffers by picking a certain option. In this case, Mikael does not want to cut his food budget. Instead, he picks to skip visiting Ouro Preto. So this represents how Mikael is losing out on seeing this site in order to eat the foods he wants while on vacation.