The flashback cost is equal to that card's mana cost
Emry, Lurker of the Loch in particular is a highly played card in formats where its legal (Pioneer, Modern, Legacy, Commander)
While on its face it seems like a major downside to playing the card, and hardly worth the cost - and it can be without the right cards in play - it pairs perfectly with the number of effects on Rise of the Floodborn cards that are triggered when cards enter or leave play, providing a easy way to put cards back in your hand, play them again and activate their effects
#5 Shimering Skies Starter Deck Emerald/Steel I dont know what it is about Steel starter decks but its hard for Lorcana to get them wrong
The same is also true of Gathan Raiders from Future Sight
these spoilers come from the Magic: The Gathering Facebook page, Wizards of the Coast employee Trick Jarett, and the English Wizards of the Coast Twitter page and Japanese Magic: The Gathering Twitter page