If an NFT is just a picture or other media then it is worth whatever people will want to pay for it, as with all art. Anything in limited supply can be attractive to collectors. Just see what an old comic can go for and that would have been pennies originally. In-game NFTs can help you make progress in the game and have a slightly different sort of value.

As for NFTs across platforms/games, that could get interesting. What if you import a machine gun into a swordplay game?

It just seems for a lot of these that you are reliant on some centralised source to keep the NFT 'alive'. Art with an ownership certificate on the blockchain may last, but if a game is shut down then you don't really have anything.

Interesting times.