Does a spell that costs generic mana get cheaper from cost reduction effects before or after additional costs are added?

When you cast a spell, the total cost is assembled in a precise sequence defined by CR 601.2f. First, you start with the spell's mana cost or an alternative cost. Second, you add any additional costs (such as kicker, or costs imposed by other effects). Third, you apply any cost increases. Fourth, you apply any cost reductions. Finally, you pay the resulting total.

This means cost reduction effects — like Goblin Electromancer reducing instant and sorcery spells by {1} — always see the full cost including any additional costs before they reduce it. They never operate before additional costs are tacked on. The order is: base cost → additional costs → increases → reductions → payment.

Importantly, cost reductions can reduce generic mana requirements but cannot reduce a spell's colored mana symbols below zero, and the total cost can never be reduced below {0} (CR 601.2f, 202.3b).

Example: You cast Overloaded Cyclonic Rift (overload cost {6}{U}{U}) while Goblin Electromancer is on the battlefield. Overload is an alternative cost, not an additional cost, but suppose instead you cast a kicked Sorcery with base cost {2}{R} and kicker {3}. The base is {2}{R}, kicker adds {3} making it {5}{R}, then Goblin Electromancer reduces the total by {1}, leaving you paying {4}{R}.

So to be precise: additional costs are added first, then reductions are applied to the combined total. This can be advantageous, since your cost reduction effectively discounts both the base and additional costs lumped together.