Have you thought about the legal implications of doing such a thing? people saw the offer on steam and bought it on a legal frame, you are revoking serial numbers because of your mistake putting the price so low, hell you are not even losing money, because most of this people will pirate your software out of anger, they did nothing wrong, this is a shady move.
Expect at least a class-action lawsuit from them.
Imagine buying a car, then, when you are about to use it comes the manufacturer and tells you to refund the car, buy it again at a price that is convenient to the manufacturer (because someone made a mistake) so you can use it. And remember you bought the car on legal terms and on a perfectly normal transaction... doesn't sound right, does it?