For example, the stipulation that "fun" does not carry a cost to another is very civilized and admirable on your part, but not in accord with the observable reality of existing social interactions.

True. I only made that stipulation because it seemed to be consistent with the tone of the comment that set me thinking about this. Anyone who wants to consider the question without the stipulation is welcome to do so.

Perhaps a better way to frame the question is this: Is fun always profitable?