Once you determine the cost to cast Sunderflock, you may activate mana abilities to pay that cost. If the greatest mana value among Elementals you control changes while activating mana abilities (probably because you sacrificed one or more Elementals), the cost to cast Sunderflock remains what you previously determined.

If an Elemental on the battlefield has in its mana cost, X is 0 for the purpose of determining its mana value.

To determine Sunderflock's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases, then apply any cost reductions. Sunderflock's mana value remains unchanged, no matter what the total cost to cast it was.

Once you announce you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to remove Elementals you control from the battlefield at that time.