I have found this article about the cost for trades. It states that:
Standard Pokémon Trades:
Regional exclusives
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
Special Trades:
Legendary Pokémon
Shiny Pokémon
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.