Twenty dollar gold coins with "copy" marked on them are worth some money but not nearly as much as the coin being replicated. They are also unlikely to be made of the same precious metals as the original coin.
Replicated coins are made so that coin enthusiasts can hold a piece of history that would normally be far too expensive or elusive to own. Many old coins were made from precious metals like gold, copper and silver and as such they are always worth the weight of the metal contained within. Replicated coins are mostly made from more common less expensive metals and are really only worth their aesthetic value.