This storage is shared across Drive, Gmail and Google+ Photos.
By comparison, Apple’s iCloud service, even on an annual basis, is much higher.
Apple charges $20 yearly for 15 GB, $50 yearly for 25 GB and $100 yearly for 55 GB (lumping in the free 5 gigabytes Apple gives to every user).
As 9to5Mac points out, Google’s service, unlike iCloud is ad-free. Still, even if you pay nothing at all Google service is clearly the better deal giving all users 15 GB for free compared to Apple’s 5 GB per Apple ID offering.