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

When you calculate the total cost of a spell, you apply all cost increases first, then all cost reductions. However, the Comprehensive Rules explicitly cap the result: the total cost can never be reduced to less than {0} (CR 601.2f). Any reduction that would push the cost below zero simply stops at zero — you cannot 'bank' extra reductions or end up with a negative cost.

This means no matter how many cost-reduction effects are stacked on a spell, the absolute floor is that the spell costs {0} generic mana (plus any remaining colored mana requirements that were not reduced). You still have to announce and 'pay' {0}, which requires no mana at all — but you cannot be paid to cast a spell through cost reduction alone.

It is important to note that colored mana symbols in a cost are treated independently. Cost-reduction effects that reduce generic mana cannot reduce colored mana requirements below zero either; each portion bottoms out at zero on its own.

Example: You cast a spell with a mana cost of {3} and have two effects each reducing its cost by {2}, for a total reduction of {4}. Instead of ending up at {-1}, the spell simply costs {0}. You cast it for free, but you receive no 'refund' mana.

Relevant rule: CR 601.2f states "The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If the mana component of the total cost is reduced to nothing, it is considered to be {0}. It can't be reduced to less than {0}."