"Mana Efficiency" of Stuns in this game (in terms of mana/second). Obviously this isn't 100% accurate since many stuns have other side effects (UB has life-drain, Frost Nova has a nice movement speed debuff) and Silence isn't a 100% stun though it doesn't provide immunity either (like TB+Erebus)
Foul Grasp: 400
Boulder Rool II: 400
Boulder Rool III: 425
Frost Nova II:500
Frost Nova III: 420
Mass Charm III: 375
Mass Charm IV: 300 (because Erebus is always best at everything D:)
Silence: 200
If you consider in a 2v2 fight Silence to cause full cooldown rotation of 2 abilities (this is even better against Oak, or Occ who have two abilities on a short cooldown that can easily be spammed) the amount of ability damage you negate is more efficient than you would have by investing your mana into a heal (heal IV is 750 mana, Silence III is 1000). It also gives you a "free" Bishop heal since Bishops typically make up for the dps a character can do esp end-game Sedna walking around with 8-9k hp (Grofflings, Nimoths, Unbreakables, Hurlington's, Girdle of the Giants, BotF, Bishops, Orb of Defiance).