Although it is certainly not one of the most expensive Gengar cards to exist, both its artwork and gameplay potential nevertheless give it its own appeal
New, format-specific support cards like Dose of Dawnglow are proving to be more efficient than older engines
Lastly, Subversive Acolyte is having its mana cost shifted from BB to 1B so it can go it in more decks, while having its toughness bumped up a point in combination with the second ability giving the creature +3+2 instead of +3+3 to make its new form still resemble Phyrexian Negator
That's knocked it down in price a fair bit, though the Ultra-Premium Collections are so costly, even their promo is worth quite a lot
Finally, 2002s Legendary Collection was the very first set to include reverse holo cards and they had a firework style pattern that remains unique to this day
Get them now while you can for cheap because they won't be for long