While most products from the Pokémon TCG right now are, to put it mildly, incredibly overpriced, only a few spectacular chase cards can command truly silly money. Case in point is Mega Gengar ex #240, which - as it's part of the upcoming Japanese expansion Mega Dream ex won't actually be available until November - is already being sold via pre-order for close to a thousand dollars.
Straight off the bat, do I think Mega Gengar ex #240 will remain one of the most expensive Pokémon cards once copies are actually in people's hands? No, I don't. The price owes to a mix of pre-launch excitement and the existing hype bubble, alongside a hefty dose of speculation based on how popular the similar Gengar VMAX #271 from Fusion Strike has become. It's just another example of how overheated the market for the Pokémon TCG is right now.
One eBay listing of the Mega Gengar ex #240 chase has already sold for $807.99, and other listings are available closer to the $700 mark. And that's for the Japanese version of the upcoming expansion, which almost always trade for less than their English-language counterparts - a daunting sign that the Special Illustrated Rare version in the English set could be farcically overvalued.
While I won't predict what the English version of the card will sell for, especially as things can change quickly from the pre-release sales to the full retail launch, I can all but guarantee the Japanese version won't stick at this price, even if Mega Dream ex is a high-value set - it's a pattern we've seen before.
Take Terastal Festival, the Japanese version of Prismatic Evolutions. It contains the same sought-after chase Umbreon ex #217 Secret Art Rare as the English set. But while the asking price for the English version is around $1000, the Japanese copy is selling for just $300 - most likely because the hype has transferred from the Japanese copy to the new hotness.

Of course, this is all prediction, and considering I once thought the hype bubble was popping (egg on my face), I'm not willing to bet money on Mega Gengar ex #240 going either way once Mega Dream ex, and its English counterpart, finally launches.
If you want to take your chances cracking booster boxes to try and lay your hands on this chase, the retailers below are good places to watch for Mega Dream ex listings when the set is available:
We'll be sharing discounts and deals for both the Japanese and English versions of the newest Pokémon set when it finally releases, so make sure to join our Wargamer Discord server to be kept in the loop as soon as they're available.