Like most Magic: The Gathering cards, the value of Kaya, Intangible Slayer 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 (Phyrexia: All Will Be One)
Bad interactions arent going to drive prices of old, bad cards up
Plus, if we wanted, we could look to Glimmer of Genius, Cathartic Reunion, Strategic Planning, or Opt for even more of this stuff
This can be done during your opponent's turn, and you will be wanting to use your Dark Rebellion Xyz Dragon to summon it after using its effect
If your opponent controls no Toon monsters, it can attack your opponent directly
I tested this on a couple of rebacked cards that I had on high for 30 seconds and nothing happened except that the card got really hot