Pickpocket Mechanic

Pickpocket X. (Whenever this creature deals combat damage to a player, reveal the top card of that player's library. If it's a nonland card with converted mana cost X or less, you may cast it without paying that card's mana cost.)
Costs {0} to add Pickpocket 2 to a creature. Pickpocket 1 is nearly useless in practice.
Costs a little less than {1} to add 3. About {1} to add 4. Jumps to {2} to add 5 or 6.
So a 1/1 for {w}{u} with pickpocket 3 is about balanced.