Buying Pokmon singles is by far the most cost-effective strategy for completing sets and decks
As speculated by TCGplayer, there could be several factors contributing to the card's massive price point, most obviously the fact that's from the recent Twilight Masquerade set which is still relatively new
If you can protect this, it shifts the game directly in your favor
You'll need the basic Pokmon in play before you can evolve it into Gengar ex
While various booster sets and expansions have seen a price hike this month, the 151 Booster Bundle has seen "an aggressive, short-term spike a 93.48% spike in the past three months, and a massive 33.35% jump in the last 30 days alone," according to IGN
Two Cautionary Tales Learn from my painful experiences--they can be helpful cautionary tales with valuable insights to reapply