OK, maybe you still have a point. What then, would be the threshold for saying: now, at this point, the costs of X are not marginal anymore but substantial enough to be taken into account?

According to Wikipedia, the best selling video game is Tetris: it has sold approximately 170 000 000¹ copies. If copying Tetris takes 1 second, the computer draws 400 W of power and electricity is rated at 30 cents/kWh, it would cost approximately $5667 to create 170 million copies of Tetris.

¹ https://en.wikipedia.org/wiki/List_of_best-selling_video_gam...

Depends on what you are thinking about ? Such cost always need to be taken into account, but IMHO in general we must always differentiate material things (that are formed from matter and its organization) and information (i.e. concept of entropy, ideas, thoughts, songs, language, jokes etc..) which have temporary physical supports but exists apart from them

Good distinction, I agree. This leads me to this: If, for example a song, is not written down anywhere, but some brains contain it, this means there some people who could reproduce this song by singing it, this song would still exist. Is a brain a temporal physical support unit in our sense?

Yes of course. As long as we have electricity in our brains we can store and process information. The very precise implementation of this would require to study biology (it's already interesting to see that we run on electricity like computers!).

Ok, cool, now, how is the number eight, for instance. Does it exist only because human brains have it in it? Or do numbers exist in the universe outside our brains? Can we answer this? This would be an important step in IP rights and all that…

It's a whole philosophical question...I would say yes maths and humans abstractions exist independently !

“Marginal costs” are the additional cost for each additional unit delivered. They can be high or low, but with modern, non-custom software they tend to be extremely low.