It's not an obvious joke. It seems closer to a puzzle in that the reader must discover that the $100 was for entertainment. This is a common class of puzzle where money changes hands between two people and results in a surprising conclusion.
A better gist may be the value of entertainment is temporary.
If instead it was just one person and they went to a movie theater. If you ignore the entertainment value it may just look like the person through away the admission cost.
