Magnus the Red

The total cost to cast a spell is locked in before you pay that cost. For example, if you control three creature tokens, including one you can sacrifice to add {C}, the total cost to cast an instant with mana cost {4}{R} is {1}{R}. Then you can sacrifice the creature when you activate mana abilities just before paying the cost.

Activating a permanent card's unearth ability isn't the same as casting it as a spell. The unearth ability is put on the stack, but the card is not. Spells and abilities that interact with activated abilities (such as Stifle) will interact with unearth, but spells and abilities that interact with spells (such as Cancel) will not.

If you activate a card's unearth ability but that card is removed from your graveyard before the ability resolves, that unearth ability will resolve and do nothing.