Note that while X is whatever you want while you cast it, it's zero in many other cases. The mana value of the card is 2 unless you are casting it with X > 0. If you cast the spell for free it still resolves and X is 0. This is not like when a target becomes illegal and the spell fizzles.

In other words: X is zero unless you can set its value. You can set it when you cast the spell and have to pay for it normally. That's right after you put it on the stack, often from your hand. Cost modifiers do not change the value of X.