How are you defining value?

I think standard economic thought defines value in terms of preferences.

Such that the value that something has to me is the supremum of how much I value any of the things I would give up in order to have it.

So, if I am willing to purchase an mp3 for $1, then I value it at at least $1. If I would also be willing to purchase it for $2, then I value it at at least $2.

If I value it at $2, and it is for sale at $1, then I would gain $1 of value in that transaction.

If the seller is willing to sell it at any price over $0.50 , then that is how much they value (not) selling it.

So then, in this transaction I would gain $1 in value, and they would gain $0.50 in value.

I think this is the standard way to think about it.

Digital goods don't have zero value. We know this because people are willing to pay for them. They just have very little cost to reproduce/transport which results in easy sharing. They're not even unique in this regard - you'll find the same easy reproducibility in books, architectural plans, art, simple circuits, manufacturing techniques, algorithms... any idea that can be expressed and transmitted easily. The problem isn't that those things are worthless (they're not), it's that knowledge can't be "owned" like physical items.

>The problem isn't that those things are worthless

It's just that they're worth a lot more when corporations lobby governments to make copying them illegal.

That doesn't make the "economically proven to have 0 value" claim true I don't think.

> since each additional copy costs the manufacturer 0

How about the first copy? That cost "0" too?

If not, are you suggesting the first buyer pay full price and everyone else get it for free?

No, but every additional copy is cheaper than the first, and asymptotically approaches 0.