Like most Pokmon cards, the value of Mewtwo & Mew-GX 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 (Unified Minds)
It fits the picnicking theme as well, making Dedenne a perfect addition to this Pokmon Pocket set doused in cuteness
Any effects that modify how the new object enters the battlefield will consider only the combined back face
While I cant find confirmation on whether these decks will contain new-to-Commander cards, the low price point makes me suspect these will be all reprints
For example, some of the most expensive cards from Temporal Forces, like Iron Crown ex (206/162), are Special Illustration Rare cards
You then use the Level X Power Big Appetite to refresh your hand and Snorlax falls Asleep again