Honestly, I think everyone has lost their collective sanity. That card is way overpriced right now, IMHO, which might make it a good time to trade it (if you need other cards).
This particular Solgaleo-GX promo has been out nearly two months. And since it's a good card in certain decks, I've traded for approx. 10-12 of them (me, my son, and another family member) so we would have playsets of at least 3. The first half-dozen, I was able to get for 6-8 value. A month ago, they were trading for 10-12. Two weeks ago, I was still trading for them at 10-12, and putting them back up at 14-15. It was easy to flip 5 or 6 this way. But in the last two weeks, they've zoomed up to the 18-20 range (asking price), as mentioned above. ...Way too much.
To answer the question about, "Will they hold value?" ...Who can say for sure what will be released, even in the near future. It's been fairly common for sets to include the Full-Art and Secret Rare of previous promos. All we know for sure is that a Full-Art version of this specific promo (same attack, etc.) has been designed and released in Japan. The Japanese card is one of the entirely gold versions, a part of their GX Ultra Shiny subset.
Edited by RobRatt, 06 December 2018 - 02:32 AM.