The cost reduction applies only to generic mana in the total cost of noncreature spells you cast
This base deck model is great to start from, and can often produce pretty decent decks by itself
There are several other possible upgrade cards, like Weapons Vendor, Sigardas Aid, Sram, Senior Edificer, and Blackblade Reforged, that could give extra value to your Limit Break deck in Magic: The Gathering
In many cases, this refunds the full cost of the spell or more, and against mana rocks, it often generates a tempo swing
This enchantment is a fixed version of Recurring Nightmare, but its still an extremely powerful addition to the Commander format that can enable all sorts of shenanigans with powerful ETB triggers