It is similar to instant kill, but with minor differences. Your opponent never paid the quanta to cast whatever they discard, and it may not be what you want to kill. Maybe you have a Hope and a stack of pillars, and you force them to discard a Steal, or you have a couple Voodoo Dolls and force them to discard a Rain of Fire.
On the other hand, it could be a Hope that you force them to discard, making it more powerful than Steal in that instance. Tough to say what it should cost, really. Especially since if it is introduced into the metagame, and you know your opponent has them, you might hold a pillar in your hand if you don't need the quanta later in the game.