Not only are new staples, like The Soul Stone, supremely expensive, but old cards also routinely spike in price
Even the tiniest bit of flaw can affect the price
The only missing piece would be some sort of healing mechanic where the -1/-1 counters are removed via another creatures activation or similar ability
So many eager collectors want to get their hands on Rosa's Encouragement, even if they don't want to risk adding her to a playable deck
The strategy would be to create an overwhelming number of effects that you can activate each turn to deal with threats and to drop some protection in the form of tokens created by walkers like Garruk, Cursed Huntsman or Elspeth, Sun's Champion
Unlike standard cards, Pokemon EX cards offer a significant advantage in battle, but they also come with a risk: if defeated, they give the opponent two prize cards instead of one