Sun & Moon is the name given to the first main expansion of cards from the Sun & Moon Series of the Pokémon Trading Card Game and many of the most expensive sun and moon cards are so playable and collectible that many players are so hyped with the idea of having them all (even when that is very difficult due to the amount of cards on each set).
At the moment 4 expansions has been released ( Guardians Rising, Burning Shadows, Shining Legends, Crimson Invasion). This sets are the first featuring Generation VII and Pokemon-GX in the card game.
Many of this cards features brand new styles and many of them add amazing new features to competitive gameplay making them very expensive.
But before you search shops that buy Pokemon cards to get rid of all your shiny gold cards learn more about their prices.