The total cost to cast Blasphemous Act is locked in before you pay that cost. For example, if there are three creatures on the battlefield, including one you can sacrifice to add to your mana pool, the total cost of Blasphemous Act is . Then you can sacrifice the creature when you activate mana abilities just before paying the cost.

Blasphemous Act's ability can't reduce the total cost to cast the spell below .

Although players may respond to Blasphemous Act once it's been cast, once it's announced, they can't respond before the cost is calculated and paid.