In that case: I think superturtles are cheap. Stay in your zone, armour up, increase your life, have guards. Cheap because it doesn't take much effort to pull off.
I beg to disagree: a simple teleport, force push, force pull can easily counter that so the turtling mage has to pay for nullifies, or spend defensive actions rather than offensive ones to make more damage. You need a lot of creatures to guard, or you'll get overwhelmed soon enough. You also lose the board control soon enough if you don't have enough creatures.
Even cheaper, easier to pull off, and very powerful, almost a classic now: the Divine Intervention/Vampiress opening. Can be countered in some ways, but takes resources, especially if the mage is nearby to support the attacks.