As posted in Official Google and Gmail blogs Google is offering a very cheap rate for people who are willing to get more storage for a price. This has become possible just because of drop in rates of storage devices. Google is not offering 20 GB for only $5. For more details visit https://www.google.com/accounts/PurchaseStorage or Official Google blogs link below.
Official Google Blog: Twice the storage for a quarter of the price: "20 GB for only $5"