Once you announce that you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't remove Sokka to change by how much Equipment spell costs are reduced.

Sokka will count himself as an Ally for his second ability.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.