5 Oct 2018
Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.
5 Oct 2018
If a permanent has {X} in its mana cost, X is considered to be 0.
5 Oct 2018
You choose whether to sacrifice a permanent (and which one to sacrifice) while Vraska's first ability is resolving. No player may take actions between the time you choose which permanent to sacrifice and the time you do so.