The mana value of a spell isn’t changed by alternative costs, cost increases, or cost reductions. For example, if you cast Thrumming Hivepool (an artifact with affinity for Slivers), its mana value is 6 no matter how many Slivers you controlled when you cast it.

For spells with {X} in their mana costs, use the value chosen for X to determine the spell’s mana value.