To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases (such as the commander tax), then apply any cost reductions (such as that of Karador). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

You must follow the normal timing permissions and restrictions of the spell you cast from your graveyard.

You must pay the costs to cast that spell. If it has an alternative cost, you may cast it for that cost instead.

Once you begin to cast the spell, losing control of Karador won't affect the spell. You can finish casting it as normal.

If multiple effects allow you to play a card from your graveyard, you must announce which permission you're using as you begin to play the card.

If you cast one creature spell from your graveyard and then have a new Karador come under your control in the same turn, you may cast another creature spell from your graveyard that turn.

If a creature card is put into your graveyard during your main phase and the stack is empty, you have a chance to cast it before any player may attempt to remove that card from your graveyard.