It's an expression generally used about dating and picking people up - if someone's out of your league, it means that the chances of picking them up are small (as in they are "too attractive" for you).
Imagine several leagues, but instead of football teams, there are people
could a native speaker please tell me if "out of someone's league" can only be said of people? Can it ever be used when speaking of something one can't afford? For example, "This restaurant is way too expensive. Completely out of my league". I believe that's wrong but still I would like to receive a confirmation.