If it's minted by an agency that helps creators do marketing, and the agency gets the creator to say "Yes, this is a legitimate NFT created on my behalf by this agency, which has been very helpful to me in understanding NFTs, I don't understand these fancy computer things," why would the value be zero?

(This exact scenario plays out all the time in the real world. People bought "Taylor Swift's" album Fearless, which was authorized by her and contained her actual voice and songwriting and paid profits to her and was in all senses legitimate, and paid well over zero dollars for it. And then over a decade later she tells her fans that she doesn't control it and she's recording her own version of the album actually owned by her.)

The question is how you prove that provenance if a niche artist has created an NFT on a niche piece of art. (And, honestly, most of the people who would want to create NFTs are going to be in this category.)

Now imagine an agency whose job was simply to create a digital signature on a work that indicated what they had done to prove a particular piece of art really was produced by the artist who then goes on to sign the NFT. If the agency has an established reputation, then that signed NFT has much more solidly established provenance than an unsigned NFT. Of course that agency has non-zero costs to establish provenance. And adds a non-zero amount to the value of the NFT.

What then, can said agency reasonably charge the artist for this service?

An NFT has no more value than a JPG.

I'll longbets anyone that says otherwise.