Whir of Invention is a rare Instant card from Aether Revolt Promos (PAER), collector number 49p. Current market price: $6.81 (foil $14.80), 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.