I have found this article about the cost for trades. It states that:
Standard Pokémon Trades:
Forms you don't have in your Pokédex (think Unown, Alolan and Castform)
Any other Pokémon not in your Pokédex that isn't Legendary or Shiny
And for the trade costs (depending on the friend level
Standard Pokémon 100 80 8 4 Shiny or Legendary (Owned) 20,000 16,000 1,600 800 Shiny or Legendary (Unowned) 1,000,000 800,000 80,000 40,000
Here is my situation:
I only have the Armoured Mewtwo, not the regular one. If I trade with someone for a normal Mewtwo, is it an owned legendary trade or an unwoned legendary trade?
I only know, that regular Pokemons that are shiny or with skin (Pikachu with hats) count as an owned Special Trade, if you only have the regular one.