There is no rule book on how to assess an NFT valuation. The metrics you use for evaluating private companies or conventional investment vehicles such as shares are simply not applicable to NFTs. Usually, the payment rolled out by the last buyer gives some indication of the value. For NFTs, however, it is hard to guess what the next buyer might pay, depending on their estimates.
Most buyers lack the skills to ascertain the value of NFTs logically and base their quotes on guesswork. For sellers too, it is hard to determine what they might end up receiving for the tokens they hold. Over time, the value of NFTs is driven by a perception over which both buyers and sellers may lack any control.