Fun is one among many "currencies" that one could potentially count one's "profits" in. Their values are all relative to each other and subjectively determined (like all prices/costs/valuations).

As a result of the above, your setup of the question itself seems partly true and partly false in a few different ways.

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. Plenty of people take pleasure in or find humor in each others misfortunes. Perhaps we can say, though, that what we decide is fun or perceive as fun "ought not" carry such costs -- or else someone's subjective valuation of something that does carry such costs is an indicator of seriously impaired personal growth on their part. Certainly an excess of such tendencies would be almost definitive of a pathologically cruel or psychopathic person. OTOH, even the most compassionate and empathetic among us will still occasionally laugh at the real or pretended misfortune of others. Look at the pratfalls Chevy Chase used to do in skits on the old Saturday Night Live episodes from way back in the day. Clearly many considered that show "fun".

And so on...

I would suggest re-examining some of your assumptions.