If you want a hard number, something thats worth at least the planeswalkers mana cost plus a number of mana equal to the number of plusses necessary to reach the ult from the starting loyalty, often more than that
it isnt a card in super high demand
And it turns out that Charlie was actually playing an important role in the unwritten rule-driven madness that is card hunting
As with its fellow swords, Sword of Fire and Ice can be made even deadlier if you can get it on something with double strike
Cards like scheming symmetry, brainstorm, crystal ball, and a number of scry cards can put your 6-9 mana spells on top of the library, so that Yuriko will bleed your opponents for massive amounts of damage each turn
For its base cost casting of four mana, this is still an okay card, especially as it's potentially giving you six mana worth of creatures back, but its sneak cost is just two mana , which feels slightly broken, especially as it also effectively turns this sorcery into an instant