A gambler's sale

Delicious Monster is holding a Gambler's Sale on its popular app Delicious Library. Every week the price goes down $5 until they've sold a secret number of copies, or until 4 weeks have elapsed. You can buy now or wait for the price to go down and risk that the sale will end before you get your purchase in. It's a bit like a clearance sale in which a retail store keeps reducing the price on items until they get rid of everything in inventory. If you wait, there's a chance that snazzy shirt you want will go down in price even more, but there's also a chance some other shopper, one who's not quite as cheap as you are, will walk off with it.