Purify should pretty clearly be 1 mana. 0 is too strong with card draw involved, but at 2 mana you're paying even more than 2 mana for a card draw, since Priest has 0 mana silence anything which is a stronger effect than silence only your own minion.
Nobody pays 2 mana for a card draw. Here are the draw values for mostly non-conditional cards. Obviously stuff like Battle Rage, Divine Favor, Northshire and Gadgetzan can blow these cards away.
Gnomish Inventor - 1.5
Arcane Intellect - 1.5 in a draw everything to win deck, 1-1.5 with added effect in tempo.
Fan of Knives - 1
Mortal Coil - 1
Slam - 1
Loot Hoarder - 1+
Azure Drake - 0.5-1, depending on how you value spell power
Power Word Shield - 0
Purify - 2+
Nobody pays 2 mana for a card draw. Here are the draw values for mostly non-conditional cards. Obviously stuff like Battle Rage, Divine Favor, Northshire and Gadgetzan can blow these cards away.
Gnomish Inventor - 1.5
Arcane Intellect - 1.5 in a draw everything to win deck, 1-1.5 with added effect in tempo.
Fan of Knives - 1
Mortal Coil - 1
Slam - 1
Loot Hoarder - 1+
Azure Drake - 0.5-1, depending on how you value spell power
Power Word Shield - 0
Purify - 2+