Most coins have no NAV (Net Asset Value) that would back their price, so to derive a fair value is incredibly hard. There are models like the one formalized by Chris Burniske (https://medium.com/@cburniske/cryptoasset-valuations-ac83479ffca7), but this is based on monetary supply theory and can only be used to derive a lower bound for the price.

The current valuations are mostly based on speculation, not the actual GDP that's being produced using them, so it gets even harder. This is why - across the field - technical analysis is what most analysts are sticking to.