Does cost reduction apply before or after additional costs are calculated?

When you cast a spell, the game follows a specific order for determining what you pay. First, you start with the spell's base mana cost (or an alternative cost if one is being used). Then, any additional costs — such as kicker, buyback, or costs imposed by effects like Thalia, Guardian of Thraben — are added on top. Only after the full cost is assembled do cost reductions get applied. This is codified in CR 601.2f and CR 601.2g.

CR 601.2f states that you determine the total cost by starting with the base cost, adding any additional costs, and then applying any cost reductions. Cost reductions can never reduce a mana cost below zero (CR 202.3c), but they operate on the final assembled cost rather than just the printed cost. This means a cost reduction can effectively offset an additional cost.

For example, suppose you cast Ancestral Vision with Suspend — but consider a simpler case: you cast a creature with a printed cost of {3}{U} while controlling an effect that says "Spells you cast cost {1} less." Your opponent has a Thalia on the battlefield, making your spells cost {1} more (an additional cost). The total cost becomes {3}{U} + {1} (Thalia) = {4}{U}, and then the cost reduction applies, bringing it back down to {3}{U}. You pay {3}{U}.

This ordering matters because it means cost reductions can cancel out additional costs imposed by opponents or by the spell itself. However, cost reductions cannot reduce the cost below {0} for the generic portion, and colored mana costs can only be reduced by effects that specifically say they reduce colored costs (CR 202.3c).

In summary: additional costs are tallied first, cost reductions are applied second, and then you pay. The sequence is always: (1) choose mode/targets, (2) determine base cost, (3) add additional costs, (4) apply reductions, (5) pay.