The game, even though it never tells you basically anything in terms of anything, does have a lot of different things to think about
Like most Pokmon cards, the value of Necrozma depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Unified Minds)
Secret Packs contain cards from a specific deck archetype, making it much easier for players to pull the cards they need rather than pulling at random from Master Packs
Of course, with that longevity and popularity, its inevitable that some cards, among the many thousands that have been released, will end up being more sought after, and therefore more expensive, than others
Proper packing is key to maintaining the value and condition of your trading cards.
D&Ds infamous Wand of Orcus has been somewhat overlooked so far, but Id definitely be picking mine up soon