Whir of Invention is a mythic Instant card from Special Guests (SPG), collector number 86. Current market price: $8.91 (foil $14.30), TCGplayer market prices via Scryfall.
Improvise (Your artifacts can help cast this spell. Each artifact you tap after you're done activating mana abilities pays for {1}.) Search your library for an artifact card with mana value X or less, put it onto the battlefield, then shuffle.