Brine Giant — ruling 1

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 (such as that of Brine Giant). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Brine Giant — ruling 2

The cost reduction ability reduces only the generic mana in Brine Giant's cost. The colored mana must still be paid.

Brine Giant — ruling 3

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 try to change by how much Brine Giant's cost is reduced.

Brine Giant : règles, fonctionnement et interactions (MTG)

Brine Giant is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Affinity for enchantments (This spell costs less to cast for each enchantment you control.)

How it works : Brine Giant

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
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 (such as that of Brine Giant). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
No other card is named in this ruling.
#2January 24, 2020· Wizards of the Coast
The cost reduction ability reduces only the generic mana in Brine Giant's cost. The colored mana must still be paid.
No other card is named in this ruling.
#3January 24, 2020· Wizards of the Coast
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 try to change by how much Brine Giant's cost is reduced.
No other card is named in this ruling.