If you apply Jodah's alternative cost to a spell with in its mana cost, X is 0.

If you cast a spell for which mana may be spent as though it were mana of any color, you may cast it for Jodah's alternative cost and still spend mana as though it were mana of any color.

Jodah's ability is an alternative cost to cast a spell. You can't combine this with other alternative costs, such as flashback. You can pay additional costs, such as kicker, in addition to this alternative cost.