I am going to assume that by the word "may" it is the creature controllers choice if they want to go searching for a creature.
If this is not the case, the casting cost should be a lot higher due to the fact that it can be a game ender if your opponent does not play with artifact creatures. Cost could be a little less if it just read artifact, but then the cost would still be higher than 4.
Also, as much as I love the idea of colorless spells, thier costs should be really high. Artifact decks have had one weakness in Magic (other than shatterstorm et. al.) and that is the inability to be a surprise instant effect coming into play during opponents turn. With so many different ways to accelerate colorless mana, a deck with solely artifacts and colorless spells would clean up (not to mention the Urza's Lands [mine, powerplant, tower]) and do so in a speed that would have a elf decks head spinning. If not a higher mana cost, then there should be some other qualifying factor to the casting of such cards. (ie, this card can only be cast if your life total is lower than your opponents)
