Earlier this month, MagicFest Richmond revealed the upcoming Magic: The Gathering Mystery Booster, which Wizards of the Coast describes as "Chaos draft and Futuresight" with a whopping 1,694 reprints and 121 copies of something new and surprising. It was revealed that each Mystery Booster pack contains one "playtest card", which could be one among a long list of cards that were never released and probably won’t ever be in the future, either. The playtest cards look more like something you'd find in an Un-set because of their weird themes and abilities. Senior Designer Gavin Verhey confirmed that these playtest cards will only be available in the Convention Edition of the Mystery Booster, and not in the one you can purchase in Wizards Play Network stores.
MTGStocks.com, a reputable website that tracks changes in trending MTG cards and their prices on the secondary market, shows some wild and diverse trends. While about three to four of the playtest cards are seen in secondary markets at less than ten USD, the value of these cards is spiking up, with some being as high as nearly $350 USD.
"Time Sidewalk" (shown above) costs $350 right now, but a lot of the playtest cards are unmarked in price so far, so some of them might even cost more than "Time Sidwalk". However, don't expect the playtest cards to retain their high prices for long since they're not legal in any MTG format but its own Limited environment. They don't even feature a legit artwork that make them look like actual Magic cards.
The WPN version of the Mystery Booster will be available on March 13, 2020; the Convention version will be available at upcoming MagicFest events and conventions where it will be featured.