To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of The Wind Crystal's first ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

In a Two-Headed Giant game, life gained by your teammate won't cause The Wind Crystal's second ability to apply, even though it will cause your team's life total to increase.

The cost reduction applies only to generic mana in the total cost of white spells you cast.

If an effect would set your life total to a specific number that's higher than your current life total, that effect causes you to gain life equal to the difference. The Wind Crystal will then double the amount of life that effect would cause you to gain. For example, if you have 3 life and an effect says that your life total "becomes 10," you would gain 7 life which The Wind Crystal doubles to 14, so your life total will actually become 17.

In the rare case where you control two of The Wind Crystal, life you gain is multiplied by four. Three of The Wind Crystal means life you gain is multiplied by eight, and so on.