Does reducing a spell's cost below zero make it free or does it stay at zero?

When you calculate the total cost of a spell, you apply cost increases first, then cost reductions. However, per Comprehensive Rule 601.2f, a total cost can never be reduced to less than zero mana. If reductions would push the mana cost below zero, it simply stays at {0}.

This means the spell becomes free — you pay nothing — but it does not go "negative" and does not give you mana back or allow you to somehow overpay in reverse. The floor is always {0}.

It is important to note that this applies to the mana component of a cost. If a spell has additional non-mana costs (like sacrificing a creature or paying life), those still must be paid in full regardless of any mana reductions. Cost reduction only affects the mana portion.

Concrete example: Suppose you cast a spell with a mana cost of {2} and you have two effects each reducing its cost by {2}, for a total reduction of {4}. Rather than owing {-2}, you simply pay {0} — the spell is free. You do not gain 2 mana or receive any other benefit from the "excess" reduction.

This rule prevents any exploit where stacking reducers would result in mana being generated as a side effect of casting a spell. The relevant rules are CR 601.2f and CR 118.7.