Cost Down is a Normal Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Cost Down is Unlimited.
YGOPRODeck lists Cost Down for these formats: GOAT, Speed Duel, OCG GOAT, Duel Links, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Cost Down appears in 2-Player Starter Set (STAX-EN013) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Cost Down is about $0.08 on Cardmarket (prices change frequently).
