Bakersbane Duo

If the cost to cast a spell is increased, decreased, or changed because of additional or alternative costs, expend counts only the mana you actually spent.

Abilities that trigger whenever you "expend N" only trigger when you reach that specific amount of mana spent on casting spells that turn. This can only happen once per turn. For example, if you've spent three mana on spells so far this turn and you control a permanent with an ability that triggers "whenever you expend 4," that ability will trigger the next time you spend at least one mana to cast a spell this turn. It won't trigger again if you spend another four mana to cast spells later in the turn.